Add coverage for all sniffs identified by testUntested()

[beporter]

The test suite now compares the full list of sniffs employed by a coding standard against those in the test sample file //~ annotations and throws an assertion failure if the sample files fail to "cover" all tests.

This acts as a crude kind of "coverage" metric of the sniffs defined in the ruleset, but right now we're only covering about 31 out of 120+ sniffs.

To resolve this, uncomment the markTestIncomplete() in LoadsysStandardTest::testUntested(). Then repeatedly run vendor/bin/phpunit and add new sample files for each remaining sniff name listed. Ideally, there should be two files for each sniff-- one confirming that incorrect style is caught by our ruleset, and another that correct behavior is not caught. The new files should be arranged in the tests/files/must/ and tests/files/must_not/ folders. Read the ## Testing section of the README for more naming details and goals.