Add coverage for all sniffs identified by testUntested()

まだ誰も着手していません。

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.

LoadsysStandardTest::testUntested() から始め、markTestIncomplete() のコメントを解除して vendor/bin/phpunit を実行し、残っている sniff 名を特定します。README の Testing セクションを読み、その後、残っている sniff 用のサンプルファイルを tests/files/must/ と tests/files/must_not/ の下に追加します。完全なルールセットの一覧が網羅され、テストスイートが成功すれば完了です。

索引モデルが issue の本文から書いたものです。