arm-auto-solutions/classes: Clean up TEST_SUITES assignment

The assignment of tests to the TEST_SUITES variable has evolved over
time to the point where it is difficult to interpret which test suites
are running in which build.

Refactor the existing assignments so that most tests are simply assigned
and :remove is only used when modifying the baremetal list for the
virtualization architecture.

No change in final test assignment or ordering.

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