RFR: 8372147: ConnectionFlowControlTest should use HttpResponse.connectionLabel()

[Volkan Yazici]

Use `HttpResponse::connectionLabel` instead of `Http2TestExchange::connectionKey` to identify the connection request-response exchange is performed on, for the latter can accidentally return the same identifier again, e.g., if a closed connection is immediately reused.

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

Commit messages:
 - Use `HttpResponse::connectionLabel` in `ConnectionFlowControlTest`

Changes: https://git.openjdk.org/jdk/pull/28534/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28534&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372147
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/28534.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28534/head:pull/28534

PR: https://git.openjdk.org/jdk/pull/28534