crypto/tls: test for timeout error using the net.Error interface

Don't do a substring search to test for a timeout error.

Fixes #14722 (maybe)

Change-Id: I4e18c749d6fd92c084a1b0b83a805119e1ae5ff2
Reviewed-on: https://go-review.googlesource.com/20403
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

tls_test.go

@@ -188,11 +188,18 @@ func TestDialTimeout(t *testing.T) {
 		t.Fatal("DialWithTimeout completed successfully")
 	}
 
-	if !strings.Contains(err.Error(), "timed out") {
-		t.Errorf("resulting error not a timeout: %s", err)
+	if !isTimeoutError(err) {
+		t.Errorf("resulting error not a timeout: %v\nType %T: %#v", err, err, err)
 	}
 }
 
+func isTimeoutError(err error) bool {
+	if ne, ok := err.(net.Error); ok {
+		return ne.Timeout()
+	}
+	return false
+}
+
 // tests that Conn.Read returns (non-zero, io.EOF) instead of
 // (non-zero, nil) when a Close (alertCloseNotify) is sitting right
 // behind the application data in the buffer.