Add PHP coverage checks

[vbelolapotkov]

Description

Add scripts for generating coverage reports and checking overall coverage.

For now I've excluded a few things:

• views (templates)
• legacy and deprecated code
• php interfaces

How to test this PR

Testing scripts:

1. Follow setup instructions in tests/README.md
2. Run tests with coverage report
3. Check overal coverage and html report locally.

Testing config:

1. Review coverage config in phpunit.xml.dist
2. Inspect excluded folders and files list.
3. Check if anything else is missing or should not be in that list.

Product impact

• Added changelog entry (or does not apply)
• Will this PR affect WooCommerce Subscriptions? yes/no/tbc, add issue ref
• Will this PR affect WooCommerce Payments? yes/no/tbc, add issue ref
• Added deprecated functions, hooks or classes to the spreadsheet

[alefesouza]

Works as expected.

✅ Coverage file is generated.
✅ npm run test:php-check-coverage shows the coverage percentage.

[vbelolapotkov]

Closing as it's no longer relevant.
Context: peNgfG-7U-p2