RFR: 8375352: java/net/httpclient/ConnectTimeoutWithProxy*.java tests fail on EC2

[Volkan Yazici]

Fix tests using `AbstractConnectTimeout` by replacing the usage of an external host (i.e., `example.com`), which unexpectedly resolves to a real IP address on certain test hosts, with a custom `ServerSocket` bootstrapped by the test.

The initial PR contains 3 commits:

1. b06f2606c88 Replace external domain with a custom `ServerSocket` (i.e., _"the fix"_)
2. b9262c7ca01 Remove the need for multiple test files
3. d95a335db93 Migrate tests to JUnit

All commits (i.e., 1, 1+2, and 1+2+3) pass tests.

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

Commit messages:
 - Migrate tests to JUnit
 - Remove the need for multiple test files
 - Replace external domain with a custom `ServerSocket`

Changes: https://git.openjdk.org/jdk/pull/29431/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29431&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375352
  Stats: 760 lines in 6 files changed: 312 ins; 448 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29431.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29431/head:pull/29431

PR: https://git.openjdk.org/jdk/pull/29431