ci: Do not repeat tests after merging MR to main

This project's GitLab repository only permits fast forward merges,
meaning that the SHA1 of the target branch (e.g. main) is always that
of the last merged merge request. Save CI resources and minimize delays
to mirroring to the public GitLab, by not rerunning tests on identical
SHA1.

Refactor rules for tests into a YAML anchor, to build and run tests when
a CI pipeline is triggered by *any* of:
* a merge request
* a release branch
* web UI

Pushes to non-release branches, including main and component auto update
branches, do not cause tests to be built and run.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>