[Feature] Reduce the amount of assertTrue/False statements

[peternied]

From #1758 (comment)

There are 1774 Assert.assertTrue(...) and and 601 Assert.assertFalse(...) statements in this codebase that are not very useful for debugging test failures. We should transition to alternatives that make test failures easier to diagnose and make tests more readable.

Consider a test case structured like this:

final String responseBody = httpClient.sendRequest('GET', '_cat');
Assert.assertTrue(body.contains("forbidden"))'

If there was a failure in this scenario it would look like

org.opensearch.security.Test > testCatApiForbidden FAILED
    org.opentest4j.AssertionFailedError: expected: <true> but was: <false>
        at app//org.junit.jupiter.api.AssertionUtils.fail(AssertionUtils.java:55)
        at app//org.junit.jupiter.api.AssertTrue.assertTrue(AssertTrue.java:40)
        at app//org.junit.jupiter.api.AssertTrue.assertTrue(AssertTrue.java:35)
        at app//org.junit.jupiter.api.Assertions.assertTrue(Assertions.java:162)
        at app//org.opensearch.security.Test.testCatApiForbidden (Test.java:107)

It would be much better if the test was structured in a way that made it clearer what was wrong such as an error like org.opentest4j.AssertionFailedError: expected: <HttpStatusCode.Forbidden> but was: <HttpStatusCode.NotFound>