add tests for goroutine local storage (#41)

I was nervous that we had zero tests in the top-level package 😅

gls_test.go

@@ -0,0 +1,70 @@
+package coroutine
+
+import "testing"
+
+func TestGLS(t *testing.T) {
+	c := make(chan int)
+
+	f := func(n int) {
+		defer close(c)
+		storeContext(getg(), n)
+
+		load := func() int {
+			v, _ := loadContext(getg()).(int)
+			return v
+		}
+
+		c <- load()
+		clearContext(getg())
+		c <- load()
+	}
+
+	go f(42)
+
+	if v, ok := <-c; !ok || v != 42 {
+		t.Errorf("unexpected first value: want=(42,true) got=(%v,%v)", v, ok)
+	}
+	if v, ok := <-c; !ok || v != 0 {
+		t.Errorf("unexpected second value: want=(0,true) got=(%v,%v)", v, ok)
+	}
+	if v, ok := <-c; ok {
+		t.Errorf("too many values received: want=(0,false) got=(%v,%v)", v, ok)
+	}
+}
+
+func BenchmarkGLS(b *testing.B) {
+	b.Run("getg", func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			for pb.Next() {
+				_ = getg()
+			}
+		})
+	})
+
+	b.Run("loadContext", func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			g := getg()
+			for pb.Next() {
+				_ = loadContext(g)
+			}
+		})
+	})
+
+	b.Run("storeContext", func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			g := getg()
+			for pb.Next() {
+				storeContext(g, 42)
+			}
+		})
+	})
+
+	b.Run("clearContext", func(b *testing.B) {
+		b.RunParallel(func(pb *testing.PB) {
+			g := getg()
+			for pb.Next() {
+				clearContext(g)
+			}
+		})
+	})
+}