tests: add a test for a failure applying patch adding file

It seems that for this case we are assuming that everything is fine, and
will apply the patch even though:

 1. The file already exists! (GNU patch seems to warn for this case)
 2. Even after confirming that we do not mind that the file already
    exists, we do not detect that there the content in that file
    _should_ cause that patch to fail.

tests/test_applier.cpp

@@ -1138,3 +1138,44 @@ PATCH_TEST(applier_multi_line_addition_from_empty_file)
 }
 )");
 }
+
+PATCH_TEST(PATCH_XFAIL_applier_add_file_but_file_already_exits_with_conflicts)
+{
+    {
+        Patch::File file("diff.patch", std::ios_base::out);
+        file << R"(
+--- /dev/null	2023-10-31 17:55:44.017202200 +1300
++++ a	2023-11-11 20:05:33.898828568 +1300
+@@ -0,0 +1,4 @@
++1
++2
++3
++4
+)";
+    }
+
+    const std::string file_contents = R"(a
+b
+c
+d
+)";
+
+    {
+        Patch::File file("a", std::ios_base::out);
+        file << file_contents;
+    }
+
+    Process process(patch_path, { patch_path, "-f", "-i", "diff.patch", nullptr });
+
+    EXPECT_EQ(process.stdout_data(), R"(The next patch would create the file a,
+which already exists!  Applying it anyway.
+patching file a
+Hunk #1 FAILED at 1.
+1 out of 1 hunk FAILED -- saving rejects to file a.rej
+)");
+
+    EXPECT_EQ(process.stderr_data(), "");
+    EXPECT_EQ(process.return_code(), 1);
+
+    EXPECT_FILE_EQ("a", file_contents);
+}