Add support to individually turn off test tasks with variables

Currently only the variable $DISABLE_CI_TESTS is supported and turns off all tests. We should also support similar variables to only turn off specific tests.

added Doing label

mentioned in commit 800fd33d

Introduced new variables:

• DISABLE_CI_TEST_BACKSTOP
• DISABLE_CI_TEST_BEHAT
• DISABLE_CI_TEST_CODESTYLE
• DISABLE_CI_TEST_PHPUNIT

closed

removed Doing label