fw_e2e_cov_summarizer: add count of unowned tests

BUG=b:165915863
TEST=ran and inspected output

Change-Id: Ia0b118f043360808bf63b67ff188b102d7813c1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/2367665
Commit-Queue: Kevin Shelton <kmshelton@chromium.org>
Commit-Queue: Greg Edelston <gredelston@google.com>
Tested-by: Kevin Shelton <kmshelton@chromium.org>
Auto-Submit: Kevin Shelton <kmshelton@chromium.org>
Reviewed-by: Greg Edelston <gredelston@google.com>