In ./scripts/test.py, readded external commands, tweaked subprocesses

- Added --exec for wrapping the test-runner with external commands, such as
  Qemu or Valgrind.

- Added --valgrind, which just aliases --exec=valgrind with a few extra
  flags useful during testing.

- Dropped the "valgrind" type for tests. These aren't separate tests
  that run in the test-runner, and I don't see a need for disabling
  Valgrind for any tests. This can be added back later if needed.

- Readded support for dropping directly into gdb after a test failure,
  either at the assert failure, entry point of test case, or entry point
  of the test runner with --gdb, --gdb-case, or --gdb-main.

- Added --isolate for running each test permutation in its own process,
  this is required for associating Valgrind errors with the right test
  case.

- Fixed an issue where explicit test identifier conflicted with
  per-stage test identifiers generated as a part of --by-suite and
  --by-case.

runners/test_runner.h

@@ -8,7 +8,6 @@
 enum test_types {
     TEST_NORMAL    = 0x1,
     TEST_REENTRANT = 0x2,
-    TEST_VALGRIND  = 0x4,
 };
 
 typedef uint8_t test_types_t;