kunit: tool: properly report the used arch for --json, or '' if not known

Before, kunit.py always printed "arch": "UM" in its json output, but...
1. With `kunit.py parse`, we could be parsing output from anywhere, so
    we can't say that.
2. Capitalizing it is probably wrong, as it's `ARCH=um`
3. Commit 87c9c163 ("kunit: tool: add support for QEMU") made it so
   kunit.py could knowingly run a different arch, yet we'd still always
   claim "UM".

This patch addresses all of those. E.g.

1.
$ ./tools/testing/kunit/kunit.py parse .kunit/test.log --json | grep -o '"arch.*' | sort -u
"arch": "",

2.
$ ./tools/testing/kunit/kunit.py run --json | ...
"arch": "um",

3.
$ ./tools/testing/kunit/kunit.py run --json --arch=x86_64 | ...
"arch": "x86_64",
Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/kunit/kunit.py

@@ -153,7 +153,7 @@ def exec_tests(linux: kunit_kernel.LinuxSourceTree, request: KunitExecRequest) -
 				test_glob = request.filter_glob.split('.', maxsplit=2)[1]
 				filter_globs = [g + '.'+ test_glob for g in filter_globs]
 
-	metadata = kunit_json.Metadata(build_dir=request.build_dir)
+	metadata = kunit_json.Metadata(arch=linux.arch(), build_dir=request.build_dir, def_config='kunit_defconfig')
 
 	test_counts = kunit_parser.TestCounts()
 	exec_time = 0.0

tools/testing/kunit/kunit_json.py

@@ -18,8 +18,8 @@ from typing import Any, Dict
 @dataclass
 class Metadata:
 	"""Stores metadata about this run to include in get_json_result()."""
-	arch: str = 'UM'
-	def_config: str = 'kunit_defconfig'
+	arch: str = ''
+	def_config: str = ''
 	build_dir: str = ''
 
 JsonObj = Dict[str, Any]

tools/testing/kunit/kunit_kernel.py

@@ -249,6 +249,8 @@ class LinuxSourceTree(object):
 			kconfig = kunit_config.parse_from_string('\n'.join(kconfig_add))
 			self._kconfig.merge_in_entries(kconfig)
 
+	def arch(self) -> str:
+		return self._arch
 
 	def clean(self) -> bool:
 		try: