Exclude third-party code from code coverage reports

[schlessera]

The code coverage currently runs over all files indiscriminately.

For meaningful code coverage reports, we should exclude third-party code, as suggested by @johnbillion in #43067.

The list can be taken from PHPCS exclusion list we already have in Core: ​https://github.com/WordPress/wordpress-develop/blob/b9b2f2a79d3506a74e3c3ce34de69ed069864dfe/phpcs.xml.dist#L16

[schlessera]

Exclusion list taken from PHPCS exclusions

[schlessera]

Attached a patch that is based on the exclusions already found in ​https://github.com/WordPress/wordpress-develop/blob/b9b2f2a79d3506a74e3c3ce34de69ed069864dfe/phpcs.xml.dist#L16 .

NOTE: For this to actually work in tests, #43067 needs to already be applied as well.

[johnbillion]

In 42665:

Build/Test Tools: Allow code coverage reports to be generated.

This corrects the attribute needed to allow PHPUnit to generate coverage reports, and also adds a blacklist of files and directories that are excluded from coverage reports.

Props schlessera, jipmoors

Fixes #43067, #43240

[johnbillion]

In 42666:

Build/Test Tools: Allow code coverage reports to be generated.

This corrects the attribute needed to allow PHPUnit to generate coverage reports, and also adds a blacklist of files and directories that are excluded from coverage reports.

Props schlessera, jipmoors

See #43067, #43240

Merges [42665] to the 4.9 branch.