Generic/DisallowYodaConditions: add code coverage ignore annotation for untestable line

[rodrigoprimo]

Description

This PR adds the @codeCoverageIgnoreStart/@codeCoverageIgnoreEnd PHPUnit annotations to ignore, for the purposes of code coverage, a line that can't be tested. I considered doing this initially when I worked on improving code coverage for this sniff in #465, but I wrongly assumed that was not an option due to the Squiz.Commenting.InlineComment.InvalidEndChar error (which is part of the standard used by PHPCS).

What I failed to notice back then is that I was adding the annotation below a comment, and this triggers the error:

// Shouldn't be possible but may happen if external sniffs are using this method.
// @codeCoverageIgnoreStart

Adding the annotation before the comment as it is done in this PR does not trigger the error because the Squiz.Commenting.InlineComment sniff will only trigger the InvalidEndChar error if the first character of the comment is a letter.

I also considered using return true; // @codeCoverageIgnore, but this is not an option because it triggers Squiz.Commenting.PostStatementComment.Found.

Related issues/external references

Part of #146

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
  • [ ] This change is only breaking for integrators, not for external standards or end-users.
• [ ] Documentation improvement

PR checklist

• [x] I have checked there is no other PR open for the same change.
• [x] I have read the Contribution Guidelines.
• [x] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
• [ ] I have added tests to cover my changes.
• [x] I have verified that the code complies with the projects coding standards.
• [ ] [Required for new sniffs] I have added XML documentation for the sniff.

[rodrigoprimo]

Here is the issue: https://github.com/PHPCSStandards/PHP_CodeSniffer/issues/560