net/http: avoid leaking the writing goroutine

The test will wait for all goroutines.
A race can occur if the writing goroutine uses the Log after the test exits.

For #58264
For #59883
For #59884

Change-Id: I9b8ec7c9d024ff74b922b69efa438be5a4fa3483
Reviewed-on: https://go-review.googlesource.com/c/go/+/490255
Reviewed-by: Damien Neil <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>

Author: fraenkel

src/net/http/serve_test.go

@@ -1744,7 +1744,13 @@ func testServerExpect(t *testing.T, mode testMode) {
 		// that doesn't send 100-continue expectations.
 		writeBody := test.contentLength != 0 && strings.ToLower(test.expectation) != "100-continue"
 
+		wg := sync.WaitGroup{}
+		wg.Add(1)
+		defer wg.Wait()
+
 		go func() {
+			defer wg.Done()
+
 			contentLen := fmt.Sprintf("Content-Length: %d", test.contentLength)
 			if test.chunked {
 				contentLen = "Transfer-Encoding: chunked"