RFR: 8352706: httpclient HeadTest does not run on HTTP2

[Daniel Jeliński]

The test was using incorrect URIs for testing, and as a result, the test was never run on HTTP2. Additionally, the HTTP2 test server was using an unconfigured SSLContext, and HTTPS was not usable. Both issues are fixed.

The test continues to pass.

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Commit messages:
 - Enable HTTP2 testing in HeadTest

Changes: https://git.openjdk.org/jdk/pull/24191/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24191&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352706
  Stats: 23 lines in 1 file changed: 7 ins; 11 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/24191.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24191/head:pull/24191

PR: https://git.openjdk.org/jdk/pull/24191