RFR: 8295944: Move the Http2TestServer and related classes into a package of its own

[Jaikiran Pai]

Can I please get a review of this change which addresses https://bugs.openjdk.org/browse/JDK-8295944?

As noted in the issue, the HttpClient testing helper infrastructure code resides in a default (unnamed) packages within the test hierarchy. The commit in this PR moves those classes into a specific package. The tests have been updated to refer to them correctly with proper package name references.
Additionally as noted in the issue, the `@modules` declaration used in the httpclient tests, were all the same and were repeated in all these tests. The commit in this  PR moves those module references into the `TEST.properties` to centralize such reference. Any test which needs additional modules can then use specific `@modules` declaration in the test definition.

No functional changes have been done in this PR.

tier1, tier2 and tier3 testing passed after these changes.

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Commit messages:
 - fix SANTest which was missed out from the refactoring
 - move out the @modules declaration from test definition, to TEST.properties
 - 8295944: Move the Http2TestServer and related classes into a package of its own

Changes: https://git.openjdk.org/jdk/pull/12160/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12160&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295944
  Stats: 1092 lines in 135 files changed: 297 ins; 335 del; 460 mod
  Patch: https://git.openjdk.org/jdk/pull/12160.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12160/head:pull/12160

PR: https://git.openjdk.org/jdk/pull/12160