RFR: 8355475: UNCTest should use an existing UNC path [v3]

[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 intended 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.

Eirik Bjørsnøs has updated the pull request incrementally with one additional commit since the last revision:

  String::replace was not helpful here

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24842/files
  - new: https://git.openjdk.org/jdk/pull/24842/files/8c719220..f9444bc5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24842&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24842&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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