[RORDEV-1229] Run e2e tests against branch-matched plugin builds

[coutoPL]

Until now the tests always ran against whatever plugin images happened to be published. There was no way to check a plugin change before it was released, and the develop suite used a shared tag that could be replaced by another run while the tests were still going.

This changes how the pipeline picks the plugins to test:

• Pull requests to master, and master itself, run against the released plugins. This tells us the shipped product still works.
• Everything else runs against plugins built from the branch under test. A new step asks both plugin repositories to build them first and waits until they are ready. Each run gets its own images, so runs can no longer interfere with each other.

The waiting and triggering logic lives in one shared script, so the plugin repositories can reuse it instead of keeping their own copies.

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The rest of the changes make the suite finish more often and fail more clearly:

• Tests are brought in line with the plugin repository's own suite, including new checks for API path restrictions and for unexpected error responses after login.
• Several tests that failed for reasons unrelated to what they were checking are now more reliable - mainly around opening the ReadonlyREST menu, waiting for Kibana to finish loading, and waiting for monitoring data to arrive.
• The APM services in the Kubernetes environment now wait for the certificates they need, instead of starting too early and timing out the whole run.
• Environment startup and each test attempt now stop after a sensible time instead of running until the job is killed, so a stuck run fails quickly and leaves usable logs.
• When something does go wrong, the run records whether Elasticsearch and Kibana were still responding, which makes it possible to tell a broken environment from a broken test.

E2E tests and changes to existing tests were ported from the ROR KBN repo.
One test was removed because the feature it covered no longer exists.

[Dzuming]

LGTM