kunit: tool: Implement listing of available architectures

To implement custom scripting around kunit.py it is useful to get a list of
available architectures. While it is possible to manually inspect
tools/testing/kunit/qemu_configs/, this is annoying to implement and
introduces a dependency on a kunit.py implementation detail.

Introduce 'kunit.py run --arch help' which lists all known architectures
in an easy to parse list. This is equivalent on how QEMU implements
listing of possible argument values.

Link: https://lore.kernel.org/r/20250415-kunit-list-v2-1-aa452cd317ae@linutronix.de
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Documentation/dev-tools/kunit/run_wrapper.rst

@@ -182,6 +182,8 @@ via UML. To run tests on qemu, by default it requires two flags:
   is ignored), the tests will run via UML. Non-UML architectures,
   for example: i386, x86_64, arm and so on; run on qemu.
 
+  ``--arch help`` lists all valid ``--arch`` values.
+
 - ``--cross_compile``: Specifies the Kbuild toolchain. It passes the
   same argument as passed to the ``CROSS_COMPILE`` variable used by
   Kbuild. As a reminder, this will be the prefix for the toolchain

tools/testing/kunit/kunit_kernel.py

@@ -14,6 +14,7 @@ import os
 import shlex
 import shutil
 import signal
+import sys
 import threading
 from typing import Iterator, List, Optional, Tuple
 from types import FrameType
@@ -201,6 +202,13 @@ def _default_qemu_config_path(arch: str) -> str:
 		return config_path
 
 	options = [f[:-3] for f in os.listdir(QEMU_CONFIGS_DIR) if f.endswith('.py')]
+
+	if arch == 'help':
+		print('um')
+		for option in options:
+			print(option)
+		sys.exit()
+
 	raise ConfigError(arch + ' is not a valid arch, options are ' + str(sorted(options)))
 
 def _get_qemu_ops(config_path: str,