kunit: tool: refactor how we plumb metadata into JSON

When using --json, kunit.py run/exec/parse will produce results in
KernelCI json format.
As part of that, we include the build_dir that was used, and we
(incorrectly) hardcode in the arch, etc.

We'll want a way to plumb more values (as well as the correct `arch`),
so this patch groups those fields into kunit_json.Metadata type.
This patch should have no user visible changes.

And since we only used build_dir in KunitParseRequest for json, we can
now move it out of that struct and add it into KunitExecRequest, which
needs it and used to get it via inheritance.

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>