Refactoring of tools/test.py

- Removed '--fast'  (As requested by Jacob / Martyn).
- Removed '--simulator' (Unused).
- Changed default targets to [ 'a64', 't32', 'a64,a32,t32'] as requested
by Jacob.
- Fixed negative_testing handling (Still disabled by default).
- tools/test.py refactoring / simplification:
  - Removed TestOption classed and used a custom argparse.Action
  instead.
  - Keep environment_options and build_options as dictionaries instead
    of strings.
  - Simplified CanRunAarch64 / CanRunAarch32.
  - Simplified ListCombinations
  - Explicitely pass variables around instead of relying on
    global variables.
  - Created a ReturnCode class.
- Cap the maximum number of threads to 32.

Takes about 31min to run on x86 (Using cccache but not regrouping the
tests at the end of the run).

Change-Id: I381b80e866974d82d7caeb2e6b7e5483f395aab2