Add an NPM command for easier PHPUnit testing on PHP 8

[desrosj]

In order to maintain PHP 5.6.20 support and add support for PHP 8, Composer is currently being used to force PHPUnit 7.x for PHP 8 (see r48957 and r49037). It would be great to have an NPM command that makes it easier to run the test suite with the Composer installed version of PHPunit.

Something like npm run test:php8. It could also be more general (maybe npm run test:php-composer) in case the Composer installed version is desired past this compatibility shim.

Also updates the .travis.yml file to use this script.

Ticket: https://core.trac.wordpress.org/ticket/51456

[desrosj]

Also update .travis.yml file to use the new NPM script.

[desrosj]

In 49099:

Build/Test Tools: Add NPM script for using the Composer installed version of PHPUnit.

The test:php NPM script runs the PHP test suite using the system installed version of PHPUnit. In some cases, the version of PHPUnit installed through Composer may be preferred.

Currently, this is true when running the test suite using PHP 8. In order to add support for PHP 8 while maintaining compatibility for PHP 5.6.20, PHPUnit 7.x must be used. But, some modifications are required to be compatible with PHP 8 (see [49037], [48957]).

This change introduces the test:php-composer NPM script, which will run the test suite using the composer installed version.

Props desrosj, SergeyBiryukov.
Fixes #51456.

Merged into core in revision 49099.

[desrosj]

In 49100:

Build/Test Tools: Ensure arguments are passed correctly for test:php-composer commands.

Follow-up to [49099].

Props swissspidy.
See #51456.