Add coverage for all sniffs identified by testUntested()

Nobody has claimed this yet.

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.

Start in LoadsysStandardTest::testUntested(), uncomment markTestIncomplete(), and run vendor/bin/phpunit to identify the remaining sniff names. Read the README's Testing section, then add sample files under tests/files/must/ and tests/files/must_not/ for the remaining sniffs; done means the full ruleset list is covered and the test suite passes.

Written by the indexing model from the issue text.