[gold] web_test.go: Add tests for Handlers.BaselineHandler() and prove that {commit_hash} in /json/expectations/commit/{commit_hash} is ignored.
[gold] web_test.go: Add tests for Handlers.BaselineHandler() and prove that {commit_hash} in /json/expectations/commit/{commit_hash} is ignored.

This CL adds two sets of tests for BaselineHandler(): those with "CommitHashSet" in their name, and those without. The former are added solely to verify that the {commit_hash} URL variable is ignored, and will be removed in a followup CL, along with said URL variable and any related code.

Bug: skia:9527
Change-Id: Icd932e6c4cfa8adeb634314ad4ae10a115931cae
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/270179
Commit-Queue: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot