[release-branch.go1.9] testing: don't fail all tests after racy test failure

The code was adding race.Errors to t.raceErrors before checking
Failed, but Failed was using t.raceErrors+race.Errors. We don't want
to change Failed, since that would affect tests themselves, so modify
the harness to not unnecessarily change t.raceErrors.

Updates #19851
Fixes #21338

Change-Id: I483f27c68c340928f1cbdef160abc0a5716efb5d
Reviewed-on: https://go-review.googlesource.com/57151
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/race/output_test.go

@@ -259,4 +259,25 @@ Goroutine [0-9] \(running\) created at:
   runtime\.newextram\(\)
       .*/runtime/proc.go:[0-9]+ \+0x[0-9,a-f]+
 ==================`},
+	{"second_test_passes", "test", "", "atexit_sleep_ms=0", `
+package main_test
+import "testing"
+func TestFail(t *testing.T) {
+	done := make(chan bool)
+	x := 0
+	go func() {
+		x = 42
+		done <- true
+	}()
+	x = 43
+	<-done
+}
+
+func TestPass(t *testing.T) {
+}
+`, `
+==================
+--- FAIL: TestFail \(0...s\)
+.*testing.go:.*: race detected during execution of test
+FAIL`},
 }

src/testing/testing.go

@@ -695,8 +695,7 @@ func tRunner(t *T, fn func(t *T)) {
 	// a call to runtime.Goexit, record the duration and send
 	// a signal saying that the test is done.
 	defer func() {
-		t.raceErrors += race.Errors()
-		if t.raceErrors > 0 {
+		if t.raceErrors+race.Errors() > 0 {
 			t.Errorf("race detected during execution of test")
 		}