bzltestutil: restore timeout signal handler

In #3920, a new mechanism to
handle test timeout was introduced. However this broke existing tests
that use SIGTERM inside.

Restore the original behavior.

go/tools/builders/generate_test_main.go

@@ -245,9 +245,23 @@ func main() {
 		// we set -test.timeout according to the TEST_TIMEOUT, we need to ignore the signal so the test has
 		// time to properly produce the output (e.g. stack trace). It will be killed by Bazel after the grace
 		// period (15s) expires.
+		//
 		// If TEST_TIMEOUT is not set (e.g., when the test binary is run by Delve for debugging), we don't
-		// ignore SIGTERM so it can be properly terminated.
-		signal.Ignore(syscall.SIGTERM)
+		// ignore SIGTERM so it can be properly terminated. (1)
+		// We do not panic (like native go test does) because users may legitimately want to use SIGTERM
+		// in tests.
+		//
+		// signal.Notify is used to ensure that there is a no-op signal handler registered.
+		// Avoid using signal.Ignore here: despite the name, it's only used to unregistered handler(s) that
+		// were previously registered by signal.Notify. See (2) for more information.
+		//
+		// (1): https://github.com/golang/go/blob/e816eb50140841c524fd07ecb4eaa078954eb47c/src/testing/testing.go#L2351
+		// (2): https://github.com/bazelbuild/rules_go/pull/3929
+		c := make(chan os.Signal, 1)
+		signal.Notify(c, syscall.SIGTERM)
+		go func() {
+			<-c
+		}()
 	}
 
 	{{if not .TestMain}}