At the moment, TESTIMAGE_AUTO and TEST_OVERALL_TIMEOUT are defined
in one place for all build jobs. This will not work with the addition
of long-running weekly jobs.

Move the definition to the Gitlab CI job config, so that:
 * The default in the base build job is to always run the tests, with
   a maximum duration of 90 minutes for "adhoc" pipelines
 * The current BSP jobs inherit the defaults
 * The Arm Auto Solutions override the test enablement with the
   existing custom logic (based on the FREQUENCY, BUILD_ARCH
   and EXTRA_IMAGE_FEATURES).

These variables use "$$" so that the string is evaluated by Bitbake
instead of Gitlab.

It is no longer necessary to disable TESTIMAGE_AUTO for
yocto-check-layer.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>