RFR: 8310645: CancelledResponse.java does not use HTTP/2 when testing the HttpClient

[Conor Cleary]

**Issue**
In CancelledResponse.java the test only checks the HttpClient against HTTP/1.1 when cancelling a BodySubscriber while receiving data. 

**Solution**
In the interest of more coverage, a new test has been created which performs the same checks against HTTP/2 to cover the client in the case of a HTTP/2 connection. A new test was created as it makes use of HttpTestServerAdapters to create the test servers. This is different to how this is performed in the original "CancelledResponse" test. There are some minor changes to how the testing is conducted with an element of randomness added to the new test.

As an open question to reviewers, the old test "CancelledResponse" and the new test "CancelledResponse2" could be merged into a single file and the HTTP/1.1 case could be updated to use more canonical testing methods as with "CancelledResponse2". Though there isn't a very pressing need for this and so it has not been included in this PR as of now.

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Commit messages:
 - 8310645: CancelledResponse.java does not use HTTP/2 when testting the HttpClient

Changes: https://git.openjdk.org/jdk/pull/14625/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14625&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310645
  Stats: 258 lines in 1 file changed: 258 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14625.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14625/head:pull/14625

PR: https://git.openjdk.org/jdk/pull/14625