Re-structured assertion code to be based on predicates.

[topbadger]

By re-structuring the assertion logic to be based upon predicates, the following enhancements have been made:

Custom Predicates

Consumers can now provide custom predicates so that they can assert on LoggingEvents in specific ways in addition to the ways previously supported by the library. This also provides a solution to #188.

Example:

assertThat(logger)
    .hasLogged(event -> event.getFormattedMessage().startsWith("First section of long log message"));

MDC Comparison Strategy

The MDC comparison strategy can now be set by consumers.

• The full MDC context isn't always relevant to the test, yet the assertions previously enforced this check. The result was that people were unable to use the fluent assertions we provide in a number of scenarios.
• The previous behaviour of requiring the MDC contents to match exactly has been retained as the default.

Example:

assertThat(logger)
    .usingMdcComparator(MdcComparator.IGNORING)
    .hasLogged(warn("Some log message"));

Enhanced Assertion Failure Messages

Assertion failure messages have been enhanced to show the list of LoggingEvents that were actually captured to make debugging the cause of the failure easier.

Example:

Failed to find event:
  LoggingEvent{level=ERROR, mdc={}, marker=Optional.empty, throwable=Optional[throwable], message='There was a problem!', arguments=[]}

The logger contained the following events:
  - LoggingEvent{level=WARN, mdc={}, marker=Optional.empty, throwable=Optional.empty, message='Some other problem', arguments=[]}
  - LoggingEvent{level=ERROR, mdc={}, marker=Optional.empty, throwable=Optional.empty, message='Yet another problem', arguments=[]}

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[sonatype-lift]

UnnecessaryLambda: Returning a lambda from a helper method or saving it in a constant is unnecessary; prefer to implement the functional interface method directly and use a method reference instead.

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Suggested change

	private static final Predicate<LoggingEvent> IGNORE_PREDICATE = event -> true;
	private static boolean ignorePredicate(LoggingEvent event){return true;}

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[sonatype-lift]

UndefinedEquals: Collection does not have well-defined equals behavior.

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