Add a jupiter-tests target to run JUnit 5 tests

[slachiewicz]

There is currently nowhere to put a JUnit 5 test: the junit task cannot run
Jupiter, and junitlauncher is only exercised by the tests ofjunitlauncher.
This adds a jupiter-tests target beside the existing run, so a Jupiter test can
be written and will actually execute.

It is deliberately additive. The 356 existing test classes stay on the junit
task, junitreport still aggregates only that task's XML, and the new target
writes its reports elsewhere. Nothing about the current reporting changes.

A single selector names the JUnit 5 tests. The junit batch excludes it and
jupiter-tests includes it, so the two runners cannot pick up the same class and
the list cannot drift out of step. UnicodeUtilTest moves across as the first
one — otherwise the target would run nothing and silently keep passing.

Failures reuse junit.failed, so check-failed and test fail the build exactly
as they already do. legacy-plain writes only to files, so the target prints the
failing reports; the fileset selects nothing when the run passed.

Why not switch the runner outright

That was the original intent, and it looks blocked. The junit run feeds
junitreport, which would mean moving to the legacy-xml listener — the subject
of 69683, 69685, 69687 and 69707, two of which report that junitreport cannot
correctly read what that listener writes. Switching wholesale would regress this
project's own test reporting. Those look worth fixing first; this target needs
none of them.

errorproperty also has no junitlauncher equivalent (70175), though it does not
matter here since both properties already map to junit.failed.

Verified

• ant jupiter-tests → Tests run: 1, Failures: 0.
• ant junit-batch -Djunit.includes="**/UnicodeUtilTest.java" → not picked up by
  the junit task; the same command with SymlinkUtilsTest does run it, so the
  exclusion is doing the work rather than the batch running empty.
• With the test deliberately broken: the assertion detail reaches the console and
  junit.failed → tests.failed → fail fires.

This change was created with AI assistance.