[blinkpy] 11/N: Mock `manifest.load_and_update`

This function was not mocked, which could cause contention on a global
cache when the `update-metadata` tests were run in parallel. The symptom
of this race was the test loader reliably omitting some metadata to be
updated (the failed tests could change based on `-j`).

See also:
  https://github.com/web-platform-tests/wpt/blob/master/tools/manifest/manifest.py

Bug: 1363752, 1299650
Test: for j in $(seq 1 5); do
Test: ./run_blinkpy_tests.py --repeat=3 -j $j;
Test: done
Change-Id: I13fc90da9c98a22783c3bb9fd56d9c563a2e41a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3896740
Reviewed-by: Weizhong Xia <weizhong@google.com>
Commit-Queue: Jonathan Lee <jonathanjlee@google.com>
Cr-Commit-Position: refs/heads/main@{#1047196}