RFR: 8353662: Add test for non-local file URL fallback to FTP

[Eirik Bjørsnøs]

Please review this test-only PR which introduces testing of the unspecified but long-standing fallback to FTP for non-local files in the 'file' URL scheme. 

This in preparation for the upcoming proposal to disable the feature by default in JDK-8353440.

Since we cannot reliably bind an FTP server to port 21, the test instead uses an HTTP proxy, binding to an ephemeral port. This menas we don't test non-proxy code paths. We still test that the FTP fallback is used, which is the key point here. (We aim to test file URL connections, not FTP URL connection internals) 

An alternative here could be to just verify that the returned URLConnection is an instance of FtpURLConnection. However, I opted for an end-to-end test here, since the amount of extra code seems reasonable.

By temporarly moving this test to tier1, I was able to confirm this test runs green also on Windows GHA.

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Commit messages:
 - Add a test verifying non-local file: URL fallback to FTP

Changes: https://git.openjdk.org/jdk/pull/24418/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24418&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353662
  Stats: 141 lines in 1 file changed: 141 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24418.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24418/head:pull/24418

PR: https://git.openjdk.org/jdk/pull/24418