Ruleset: remove test specific code

[jrfnl]

Description

Ruleset: remove test specific code

The PHPCS 2.7.1 release introduced a change to how sniff tests are run.
Originally, sniff tests would be run in the context of their ruleset. This could lead to parts of the sniff being untestable, like if the ruleset would exclude a error code or contained custom property settings.

As of PHPCS 2.7.1, sniff tests are run in isolation and no longer take the ruleset of the standard they belong to into account, which solves the above problem.

This change was introduced via commit 3df85dc.

Unfortunately, the way this change was made was via a change to the Ruleset class, not by changing the test framework.

It basically meant that the Ruleset class would now behave differently when the following two conditions were met:

1. It was being used in a test context (PHP_CODESNIFFER_IN_TESTS constant defined).
2. Sniff restrictions were applied via the Config, either by using the CLI --sniffs argument or by setting the Config::$sniffs property directly.

In other words, this now created a new problem when testing parts of the Ruleset class which would need the Config::$sniffs property to be set.
It also makes writing various other tests for the framework more difficult as there are plenty of times when tests would benefit from the Config::$sniffs property being respected. In other words, this led to work-arounds being needed in various other places in the tests, either by emulating the sniff selection or by bypassing the Ruleset conditions and doing the sniff selection in a test specific ruleset XML fixture file.

Aside from the above, it also makes the barrier to entry for contributors to write tests for PHPCS that much higher as the behaviour is non-intuitive.

All in all, time to get rid of the condition in the Ruleset class.

Now, of course, we do still want to test sniffs in isolation, so to do that the AbstractSniffUnitTest class has been updated to on-the-fly create ruleset XML files which only include a single sniff without any customizations.

I considered using php://memory or php://temp instead of writing the file to disk, however, neither of those work with file_get_contents(), which is used to read the XML ruleset files in the Ruleset class, so unfortunately, those temporary file writes cannot be avoided without making bigger changes, which is outside the scope of this PR.

Now, I wondered about the performance impact of writing a file to disk for every sniff being tested, and while there is a small, but noticeable, difference when running on Windows (20 vs 27 seconds for --filter Standards), IMO this difference is acceptable when offset against the hours and hours of dev time lost trying to debug why perfectly valid tests weren't working due to --sniffs not being respected in the tests.

And to be sure, I've also tested the new test isolation mechanism with an external standard which is using the PHPCS native test framework and it looks to be working fine.

Note: even though the AbstractSniffUnitTest will clean up the temporary file after each test, if the test run would crash, it could be possible for the file to remain on disk. With this in mind, the file name for the temporary file has been added to the .gitignore file.

Related to #966

Tests: remove work-arounds for --sniffs not being respected

Now that the --sniffs CLI argument will be respected in the tests, a number of work-arounds previously introduced can be removed.

Note: there may be a few more work-arounds which can be removed, like removing some fixture rulesets in favour of test inline sniff selection, but this should be a good first step.

Suggested changelog entry

Changed:
The Ruleset class no longer has special behaviour when used in a test context.