tests: Refactor the Gem5Fixture to derive from UniqueFixture

Gem5Fixture is used to define a fixture for building the gem5
binary. Most tests are expected to define their own Gem5Fixture,
however, as some might depend on the same binary (e.g.,
./build/ARM/gem5.opt), they will try to re-define a fixture for the
same target. This patchset changes Gem5Fixture to derive from
UniqueFixture.

In addition, this patchset changes the way global fixtures are
discovered to work with the new Gem5Fixture class. Instead of
enumerating them when test definitions are loaded, we do so after the
tests have been filtered according to specified tags (e.g., include
opt variant, exclude fast, debug variants).

Change-Id: Ie868a7e18ef6c3271f3c8a658229657cd43997cb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19251
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

tests,ext: Add a new testing library proposal

The new test library is split into two parts: The framework which resides
in ext/, and the gem5 helping components in /tests/gem5.

Change-Id: Ib4f3ae8d7eb96a7306335a3e739b7e8041aa99b9
Signed-off-by: Sean Wilson <spwilson2@wisc.edu>
Reviewed-on: https://gem5-review.googlesource.com/4421
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>