RFR: 8301243: java/net/httpclient/http2/IdleConnectionTimeoutTest.java intermittent failure

[Conor Cleary]

### Description ###
Intermittent failures of this test are observed on frequent `HttpClient` test runs. The test checks that the same connection is not used twice for two seperate requests if an Idle Connection Timeout occurs by verifying that the client-side port does not use the same port. It also verifies that when an Idle Connection Timeout does not occur, the same connection is used by verifying that the port used in both requests is the same.

The issue here is that there is no guarantee that the ports used will not be the same for when an Idle Connection Timeout occurs and so the test will/does fail intermittently.

### Summary of Changes & Justification ###
Instead of comparing the post numbers of the connections used for each request in all test cases, the connections themselves are now compared with calls to `hashCode()` like so. The connection instances themselves are accessed by using a customised `ExchangeSupplier` for the `Http2TestServer`.

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Commit messages:
 - 8301243: java/net/httpclient/http2/IdleConnectionTimeoutTest.java intermittent failure

Changes: https://git.openjdk.org/jdk/pull/12457/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12457&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301243
  Stats: 65 lines in 1 file changed: 41 ins; 1 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/12457.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12457/head:pull/12457

PR: https://git.openjdk.org/jdk/pull/12457