Enhance result reporting for assertTrue() and assertFalse()

Problem/Motivation

Debugging SimpleTest failures is difficult when the test assertions do not provide enough information or when there are dozens of similar assertions in the same test. The test result list is often hard to scan.

Proposed resolution

Improve the developer experience and usability of tests by enhancing result reporting with different icons for each type of assertion when the assertion message is insufficient.

Remaining tasks

• The attached patch implements this functionality for assertTrue() and assertFalse() when no specific assertion message is provided.
• To test the effect of this change, I suggest applying this patch locally and running the "Argument validator" test in the Views Plugins test group.
• Be sure to expand and read the detailed results for the test.

Note that the patch is a work in progress and will need cleanup. For now, it bends the existing API rather than introducing API changes, so that we can evaluate this proposal before proceeding. I recommend reviewers test the patch manually before doing a code review.

@heyrocker also contributed to this patch. (I've no design sense, and heyrocker was able to assist me with some presentational elements thanks to his journalism degree.)