[Labqual] Making servo_type flag optional.

Cleaning up the code and making servo_type flag optional as it is only used for a diagnostic check.

BUG=b:341384475
TEST=Local testing with a lab DUT.

Change-Id: Iaf29b94ef9130c9f326e88ed088b5b38125f9a87
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/5547953
Commit-Queue: Gowri Denduluri <gowriden@google.com>
Tested-by: Gowri Denduluri <gowriden@google.com>
Reviewed-by: Garry Wang <xianuowang@chromium.org>