RFR: 8346463: Add test coverage for deploying the default provider as a module

[Alan Bateman]

test/jdk/java/nio/file/spi/SetDefaultProvider.java tests deploying with a custom file system provider as the default provider. The test is moved to JUnit and is updated with new tests that deploy the custom default provider on the module path. To support the different deployments, TestProvider is moved into a named package and module.

In preparation for JDK-8331467, a test is added to create a run-time image that contains the custom default file system provider.

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

Commit messages:
 - Update UnixDomainSocketAddress/AddressTest.java
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/22797/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22797&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346463
  Stats: 224 lines in 8 files changed: 77 ins; 49 del; 98 mod
  Patch: https://git.openjdk.org/jdk/pull/22797.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22797/head:pull/22797

PR: https://git.openjdk.org/jdk/pull/22797