Rely on standard vendor/bin/phpunit via CLI instead.
See https://github.com/silverstripe/silverstripe-framework/issues/4254
- Not disabling xdebug. That should be harmful, and is actively harming test development
- No longer able to specifically skip tests via the `SkipTests` request parameter. Use phpunit.xml groups and the `--exclude-group` CLI argument instead
- No longer able to specify multiple comma-separated module folders. use phpunit.xml groups and the `--group` CLI argument instead
- Not explicitly calling `SSViewer::flush_template_cache()` (was never the case on running `phpunit` CLI anyway, and shouldn't be required any more)
It is now provided automatically via composer. Also, replace the section
about installing PHPUnit with a slightly expanded section on
conveniently invoking phpunit.
