RFR: 8342836: Automatically determine that a test in the docs test root is requested [v2]

[Nizar Benalla]

> Please review this patch to automatically set `TEST_DEPS` to `docs-jdk` when testing the documentation.
> 
> This comes as a followup to https://github.com/openjdk/jdk/commit/07f550b85a3910edd28d8761e2adfb8d6a1352f6 , as review comments suggested better aways of adding dependencies and to not require users to use a verbose `make test TEST=docs_all TEST_DEPS=docs-jdk`.
> 
> I have verified that `make test-docs_all` does indeed build the docs, `make test TEST=docs_all` does not but the former is clear and short enough to be used.
> 
> TIA.

Nizar Benalla has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - add test-docs to ALL_TARGETS based on magnus' explanation
 - Merge remote-tracking branch 'upstream/master' into docs-test-deps
 - set an alias for testing docs

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/22239/files
  - new: https://git.openjdk.org/jdk/pull/22239/files/ed4de20b..0cab4f9b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22239&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22239&range=00-01

  Stats: 48249 lines in 1136 files changed: 24961 ins; 17884 del; 5404 mod
  Patch: https://git.openjdk.org/jdk/pull/22239.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22239/head:pull/22239

PR: https://git.openjdk.org/jdk/pull/22239