Add helpers for running and viewing pcov code coverage

[markmetcalfe]

This improvement adds the pcov extention to the PHP 7.3-8.4 containers, and some helper aliases unit_pcov, set_coverage to the PHP container shells to make running code coverage and viewing reports easier.

This supports Totara 14 and above (as 13 and below use PHPUnit 8 which has a different configuration)

To test this:

1. Checkout this PR
2. Build each of the containers changed in this PR (tbuild nginx php-7.3 php-7.4 php-8.0 php-8.1 php-8.2 php-8.3 php-8.4)
3. Re-up the nginx container via tup nginx
4. Checkout t14-release
5. Re-up and log into the PHP container via tup php && tzsh php
6. Test installing phpunit via installunit
7. Test setting a path to run coverage on works via set_coverage server/lib server/totara/core (which will add coverage paths for server/lib and server/totara/core)
8. Run unit_pcov path/to/test.php and make sure there are no errors
9. Make sure you are able to open and view the coverage report located at http://coverage-report.localhost (ie outputted in the terminal)
10. Make sure you are able to run tests normally without coverage via unit path/to/test.php without any issues
11. Repeat steps 4 to 10 but with t19-release

Will update the docker-dev wiki and the public developer docs on coverage once this is tested and merged

[LinnyTheDuck]

Tested and working, LGTM