Less strict threshold for timing dependent test

- Add case missed in 5af46787893a918d6589f4ee1e7f6a76d28574b4

psiphon/common/obfuscator/passthrough_test.go

@@ -123,7 +123,7 @@ func TestTLSPassthrough(t *testing.T) {
 				timeDiff = -timeDiff
 			}
 
-			if timeDiff.Microseconds() > 100 {
+			if timeDiff.Microseconds() > 500 {
 				t.Fatalf("unexpected elapsed time difference")
 			}
 		})