test-boot: run in parallel

--quit-after-boot: fix for gem5, update path to gem5.sh

Improve the printing of results and errors:

- remove newlines from IDs at the end for ./test-boot
- remove newlines from progress for __call__ commands and don't print executed commands at all,
  otherwise there are too many lines per test and it is hard to tell what is going on
- print backtraces for any exception in the threads (bugs while developing this code)

Tests across different archs and emulators are still not running in parallel,
which is a huge loss. TODO.

thread_pool: introduce with API. This was motivate by test-boot, I've had enough
of doing separate error handling for each loop type! Greatly dries up the code, awesome.

common: make --all-emulators work properly with native hopefully for the last time,
./test-baremetal was still failing.

gem5: don't pass --command-line for baremetal. Maybe later we can use it to actually
pass command line arguments to main()? To be seen.

test-gdb

@@ -57,10 +57,10 @@ found by searching for the Python test files.
                     test_source_base = os.path.relpath(base, self.env['root_dir'])
                     common_args = self.get_common_args()
                     common_args['baremetal'] = test_source_base + ext
-                    test_id_string = self.test_setup(test_source_base)
                     run_args = common_args.copy()
                     run_args['gdb_wait'] = True
                     run_args['background'] = True
+                    test_id_string = self.test_setup(run_args, test_source_base)
                     run_thread = threading.Thread(target=lambda: run(**run_args))
                     run_thread.start()
                     gdb_args = common_args.copy()