RFR: 8355475: UNCTest should use an existing UNC path

[Eirik Bjørsnøs]

Please review this test-only PR which updates `UNCTest.java` to use a UNC path which is known to exist.

The test currently uses the file URL `file://jdk/LOCAL-JAVA/jdk1.4/win/README.txt`, but since this is unlikely to resolve to an existing UNC path on any CI server, the test doesn't really verify what it's supposed to.

This PR changes the test to instead use the path `file://computername/C$/Windows`, which should always be available unless Administrative Shares has been disabled. We detect this case by using File::exists for the path and simply skip the test if it does not exist.

I forced this test to run in tier1 on GHA and verified that it ran successfully, without being skipped. Meaning Administrative Shares is enabled in the Windows GHA instance.

A test run in Oracle's CI verifying a successful run (without skipping) would be welcome.

This is a test only PR, `noreg-self` has been added in the JBS.

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Commit messages:
 - UNCTest should use an existing UNC path

Changes: https://git.openjdk.org/jdk/pull/24842/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24842&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355475
  Stats: 33 lines in 1 file changed: 30 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/24842.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24842/head:pull/24842

PR: https://git.openjdk.org/jdk/pull/24842