RFR: 8373537: Migrate "test/jdk/com/sun/net/httpserver/" to null-safe "SimpleSSLContext" methods [v3]

[Volkan Yazici]

> Migrates `test/jdk/com/sun/net/httpserver/` to null-safe `SimpleSSLContext` methods introduced in [JDK-8372661], which is a prerequisite for this change set.

Volkan Yazici has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:

 - Merge remote-tracking branch 'upstream/master' into simpleSslHs
 - Move `SSLContext::setDefault` to static blocks
 - Restore `test/jdk/com/sun/net/httpserver/` changes
 - Make sure `get()` returns the same instance
 - Reverted all changes and only kept `SimpleSSLContext` enhancements
 - Overhaul `SimpleSSLContext` and its usages

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Changes: https://git.openjdk.org/jdk/pull/28800/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28800&range=02
  Stats: 52 lines in 15 files changed: 9 ins; 22 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/28800.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28800/head:pull/28800

PR: https://git.openjdk.org/jdk/pull/28800