kunit: tool: show list of valid --arch options when invalid

Consider this attempt to run KUnit in QEMU:
$ ./tools/testing/kunit/kunit.py run --arch=x86

Before you'd get this error message:
kunit_kernel.ConfigError: x86 is not a valid arch

After:
kunit_kernel.ConfigError: x86 is not a valid arch, options are ['alpha', 'arm', 'arm64', 'i386', 'powerpc', 'riscv', 's390', 'sparc', 'x86_64']

This should make it a bit easier for people to notice when they make
typos, etc. Currently, one would have to dive into the python code to
figure out what the valid set is.
Signed-off-by: NDaniel Latypov <dlatypov@google.com>
Reviewed-by: NDavid Gow <davidgow@google.com>
Reviewed-by: NBrendan Higgins <brendanhiggins@google.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>

tools/testing/kunit/kunit_kernel.py

@@ -198,8 +198,9 @@ def get_source_tree_ops(arch: str, cross_compile: Optional[str]) -> LinuxSourceT
 		return LinuxSourceTreeOperationsUml(cross_compile=cross_compile)
 	elif os.path.isfile(config_path):
 		return get_source_tree_ops_from_qemu_config(config_path, cross_compile)[1]
-	else:
-		raise ConfigError(arch + ' is not a valid arch')
+
+	options = [f[:-3] for f in os.listdir(QEMU_CONFIGS_DIR) if f.endswith('.py')]
+	raise ConfigError(arch + ' is not a valid arch, options are ' + str(sorted(options)))
 
 def get_source_tree_ops_from_qemu_config(config_path: str,
 					 cross_compile: Optional[str]) -> Tuple[

tools/testing/kunit/kunit_tool_test.py

@@ -289,6 +289,10 @@ class LinuxSourceTreeTest(unittest.TestCase):
 				pass
 			kunit_kernel.LinuxSourceTree('', kunitconfig_path=dir)
 
+	def test_invalid_arch(self):
+		with self.assertRaisesRegex(kunit_kernel.ConfigError, 'not a valid arch, options are.*x86_64'):
+			kunit_kernel.LinuxSourceTree('', arch='invalid')
+
 	# TODO: add more test cases.