Fix the logic to detect manual tests in testharnessreport.js

Wpt upstream detects test type using the code in [1]. Update chromium
side code to only check the last element after '-' is "manual".

Manual tests are all skipped as of now.

[1] https://github.com/web-platform-tests/wpt/blob/88149528a94fdf769c29e129f8979859e39c339c/tools/manifest/sourcefile.py#L207-L208

Bug: 40188001
Change-Id: I53f13d59878ef04997c2f5dbb5686dd44a8017ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5393581
Reviewed-by: Jonathan Lee <jonathanjlee@google.com>
Auto-Submit: Weizhong Xia <weizhong@google.com>
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1277879}