Removing assertion in asynchronous operation to avoid flaky test

[mkl201]

What is the purpose of this PR

• This PR fixes a flaky test to avoid an assertion on an asynchronous operation uk.co.flax.luwak.TestCachePurging#testBackgroundPurges

Reproduce the test failure

• The test fails non-deterministically when execution delays occur. To observe a test failure, the unmodified test can be re-executed many times. The likelihood of observing a test failure increases when the test executes on a slow machine. To deterministically reproduce the test failure, introduce an execution delay:

@Test
public void testBackgroundPurges() throws IOException, InterruptedException, UpdateException, Exception {

    QueryIndexConfiguration config = new QueryIndexConfiguration().setPurgeFrequency(1, TimeUnit.SECONDS);
    try (Monitor monitor = new Monitor(new LuceneQueryParser("field"), new MatchAllPresearcher(), config)) {

        assertThat(monitor.getQueryCacheStats().lastPurged).isEqualTo(-1);

        for (int i = 0; i < 100; i++) {
            monitor.update(newMonitorQuery(i));
        }
        monitor.deleteById("5");
        Thread.sleep(600L);
        assertThat(monitor.getQueryCacheStats().queries).isEqualTo(99);
        assertThat(monitor.getQueryCacheStats().cachedQueries).isEqualTo(100);

        TimeUnit.SECONDS.sleep(2);
        assertThat(monitor.getQueryCacheStats().queries).isEqualTo(99);
        assertThat(monitor.getQueryCacheStats().cachedQueries).isEqualTo(99);
        assertThat(monitor.getQueryCacheStats().lastPurged).isGreaterThan(0);
    }
}

Expected result:

• The test should run successfully in all runs.

Actual result:

• In 57 out of 100 repetitions of the test suite, the test fails with:

org.junit.ComparisonFailure: expected:<[100]> but was:<[99]>

Why the test fails

Fix

• Remove the assertions on an asynchronous operation.