Consider Unified Builder Test Helper

[vjkoskela]

As per @speezepearson feedback consider a single Builder test helper that performs all builder related tests. If more common testing is added then modifying commons would modify all tests that use that common entry point.

e.g.

public class ThreadLocalBuildableTestHelper {
    public static void testAllInvariants(
        final Class<T> targetObjectType
        final Supplier<ThreadLocalBuilder<T, ?>> builderSupplier
    ) {
            BuildableTestHelper.testBuild(
                    builderSupplier.get(),
                    targetObjectType);

            ThreadLocalBuildableTestHelper.testResetbuilderSupplier.get());

            final String asString = builderSupplier.get().build().toString();
            Assert.assertNotNull(asString);
            Assert.assertFalse(asString.isEmpty());

    }
}

From:
https://github.com/ArpNetworking/metrics-aggregator-daemon/pull/195/files