Save test_name and experiments to config

Arguments are passed from parents and parsed correctly. But it was never
saved to config for use.

This CL saves two additional init-time optional arguments to config so
it can be used.

BUG=b:439733487
TEST=./bisect_cr_localbuild_internal.py init --session testsession
TEST=--experiments gemini_bisect --test-name tast.arc.Boot --old
TEST=b056c86f95925bb7f6037244cb640d45d9aebaa4 --new
TEST=e9db94a7439c3feeba68acd5cd7dbe178bce3e2b --chrome-root
TEST=~/bisect-workdir/test/chrome --chrome-mirror
TEST=~/git-mirrors-ext4/chrome

Change-Id: I27cf4da5ae99124d8cbd1c89dc799de9f70f094c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/7532766
Auto-Submit: Qijiang Fan <fqj@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Qijiang Fan <fqj@chromium.org>
Tested-by: Qijiang Fan <fqj@chromium.org>