Add XCTAsyncTest and XCTAssertThrowsError from gRPC Swift (#493)

NOTICE.txt

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+
+                            The AsyncHTTPClient Project
+                            ===========================
+
+Please visit the AsyncHTTPClient web site for more information:
+
+  * https://github.com/swift-server/async-http-client
+
+Copyright 2017-2021 The AsyncHTTPClient Project
+
+The AsyncHTTPClient Project licenses this file to you under the Apache License,
+version 2.0 (the "License"); you may not use this file except in compliance
+with the License. You may obtain a copy of the License at:
+
+  https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+License for the specific language governing permissions and limitations
+under the License.
+
+Also, please refer to each LICENSE.txt file, which is located in
+the 'license' directory of the distribution file, for the license terms of the
+components that this product depends on.
+
+---
+
+This product contains derivations of various scripts from SwiftNIO.
+
+  * LICENSE (Apache License 2.0):
+    * https://www.apache.org/licenses/LICENSE-2.0
+  * HOMEPAGE:
+    * https://github.com/apple/swift-nio
+
+---
+
+This product contains a derivation of "XCTest+AsyncAwait.swift" from gRPC Swift.
+
+  * LICENSE (Apache License 2.0):
+    * https://www.apache.org/licenses/LICENSE-2.0
+  * HOMEPAGE:
+    * https://github.com/grpc/grpc-swift
+
+---
+
+This product contains a derivation of the Tony Stone's 'process_test_files.rb'.
+
+  * LICENSE (Apache License 2.0):
+    * https://www.apache.org/licenses/LICENSE-2.0
+  * HOMEPAGE:
+    * https://github.com/tonystone/build-tools/commit/6c417b7569df24597a48a9aa7b505b636e8f73a1
+    * https://github.com/tonystone/build-tools/blob/master/source/xctest_tool.rb
+
+---

Tests/AsyncHTTPClientTests/XCTest+AsyncAwait.swift

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+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the AsyncHTTPClient open source project
+//
+// Copyright (c) 2021 Apple Inc. and the AsyncHTTPClient project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of AsyncHTTPClient project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+/*
+ * Copyright 2021, gRPC Authors All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#if compiler(>=5.5)
+import XCTest
+
+extension XCTestCase {
+    @available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+    /// Cross-platform XCTest support for async-await tests.
+    ///
+    /// Currently the Linux implementation of XCTest doesn't have async-await support.
+    /// Until it does, we make use of this shim which uses a detached `Task` along with
+    /// `XCTest.wait(for:timeout:)` to wrap the operation.
+    ///
+    /// - NOTE: Support for Linux is tracked by https://bugs.swift.org/browse/SR-14403.
+    /// - NOTE: Implementation currently in progress: https://github.com/apple/swift-corelibs-xctest/pull/326
+    func XCTAsyncTest(
+        expectationDescription: String = "Async operation",
+        timeout: TimeInterval = 30,
+        file: StaticString = #filePath,
+        line: UInt = #line,
+        function: StaticString = #function,
+        operation: @escaping @Sendable () async throws -> Void
+    ) {
+        let expectation = self.expectation(description: expectationDescription)
+        Task {
+            do {
+                try await operation()
+            } catch {
+                XCTFail("Error thrown while executing \(function): \(error)", file: file, line: line)
+                Thread.callStackSymbols.forEach { print($0) }
+            }
+            expectation.fulfill()
+        }
+        self.wait(for: [expectation], timeout: timeout)
+    }
+}
+
+@available(macOS 12, iOS 15, tvOS 15, watchOS 8, *)
+internal func XCTAssertThrowsError<T>(
+    _ expression: @autoclosure () async throws -> T,
+    verify: (Error) -> Void = { _ in },
+    file: StaticString = #file,
+    line: UInt = #line
+) async {
+    do {
+        _ = try await expression()
+        XCTFail("Expression did not throw error", file: file, line: line)
+    } catch {
+        verify(error)
+    }
+}
+
+#endif // compiler(>=5.5)