ci: Optimize running time

* Run `build` and `test` job in single job.
  CI often waits for runner allocation. Combining `build` and `test`
should improve the allocation time as the number or runners will be
reduced.

* Run combinations of TOOLCHAIN, PLATFORM, TEST in a single job.
  Fetching dependent CMake modules for a test is costly. Combining these
combinations will let CI to download the CMake module just once. Hence,
reducing the overall build time.

Signed-off-by: Saheer Babu <saheer.babu@arm.com>