RFR: 8253424: Add support for running pre-submit testing using GitHub Actions [v4]

[Robin Westberg]

> A few days ago I posted an initial version of the necessary configuration required to run pre-submit build and tests
> for JDK main-line contributions using GitHub Actions [2] and the free tier [3] available to everyone working with open
> source repositories. I've incorporated the feedback into an updated version that I believe is ready to be integrated.
> If this is integrated into the `master` branch, future branches created and updated in personal forks will build and
> run the basic tier 1 tests as described in this configuration, on Linux, Windows and macOS (all on x64). It's of course
> possible for any contributor to opt out fully or partially of these automatic runs in a few different ways.  To opt out
> completely, a contributor can simply disable GitHub Actions on their personal fork, and no further jobs will be
> executed. Another option is to add a repository secret [4] with the name `JDK_SUBMIT_FILTER` set to any value. If this
> is set, only branches prefixed with `submit/` will be subject to automatic build and test. This can also be further
> refined by adding a repository secret named `JDK_SUBMIT_PLATFORMS` with a value such as `Linux x64, Windows x64` to
> limit automatic build and test to these two platforms. It will still be possible to run the tests on any branch and/or
> platform by manually triggering the workflow.  To see what this looks like in practice, an example run can be found
> here: https://github.com/rwestberg/jdk/actions/runs/265131985 (note that there is currently a failing test on Windows
> which is tracked by JDK-8249095, which should probably be resolved before this change is integrated).  Best regards,
> Robin  [1] https://mail.openjdk.java.net/pipermail/jdk-dev/2020-September/004736.html [2]
> https://github.com/features/actions [3]
> https://docs.github.com/en/actions/getting-started-with-github-actions/about-github-actions#usage-limits [4]
> https://docs.github.com/en/actions/reference/encrypted-secrets

Robin Westberg has updated the pull request incrementally with one additional commit since the last revision:

  Update boot JDK to 15

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/284/files
  - new: https://git.openjdk.java.net/jdk/pull/284/files/496d896c..de5074fe

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=284&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=284&range=02-03

  Stats: 10 lines in 1 file changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/284.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/284/head:pull/284

PR: https://git.openjdk.java.net/jdk/pull/284