RFR: 8353662: Add test for non-local file URL fallback to FTP [v2]

[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.

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

  Do not assume that the loopback address is IPv4

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24418/files
  - new: https://git.openjdk.org/jdk/pull/24418/files/f235d79f..18b449de

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24418&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24418&range=00-01

  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 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