RFR: 8261625: Add `Elements.isAutomaticModule(ModuleElement)`

[Joe Darcy]

Addition of a small convenience method, with a default implementation, to Elements.

Most of the change is testing infrastructure. The negative cases, such as named modules, are tested in the style of other annotation processing tests for Elements utilities.

An automatic module comes from a jar file, so an API reading from a jar file is needed. To avoid piping in the awkward plumbing needed to do that directly in the annotation processing tests, I updated an existing automatic modules test which uses the tool box facility. My additions to the tool box might not be the done in the most idiomatic style for that API; happy to have guidance on how to proceed better there.

Assuming this general approach is agreed to, I'll update the imports on AutomaticModules.java to condense the text of the annotation processor source.

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Commit messages:
 - Add test coverage.
 - Merge branch 'master' into 8261625
 - 8261625

Changes: https://git.openjdk.java.net/jdk/pull/3382/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3382&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261625
  Stats: 205 lines in 5 files changed: 199 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3382.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3382/head:pull/3382

PR: https://git.openjdk.java.net/jdk/pull/3382