gtestutils: allow specifying mode through the environment
Unlike for example Automake, Meson makes it somewhat hard to pass different command line arguments to the tests according to whether they should be run in quick mode or slow mode.
This merge request allows the user to specify the test mode using a G_TEST_MODE environment variable, which can also be turned into a Meson test setup using for example
add_test_setup('slow', env: ['G_TEST_MODE=slow'])and used with meson test --setup=slow