private static void verifyAndSetFlagsCleared(boolean newValue) {
            if (flagsCleared == newValue) {
                throw new IllegalStateException("clearFlagsForTesting called while already cleared - running tests in parallell!?");
            }
            flagsCleared = newValue;
        }
Used to implement a simple verification that Replacer is not used by multiple threads.
For instance two different tests running in parallel cannot both use Replacer.
flags/src/main/java/com/yahoo/vespa/flags/Flags.java