RFR: 8373538: Migrate all tests to null-safe "SimpleSSLContext" methods

[Volkan Yazici]

1. [JDK-8372661] introduced null-safe static factory methods to `SimpleSSLContext`
2. [JDK-8373515] migrated `test/jdk/java/net/httpclient/` to these new methods
3. [JDK-8373537] migrated `test/jdk/com/sun/net/httpserver/` to these new methods
4. This PR migrates all remaining `SimpleSSLContext` usages, *and* removes nullable `SimpleSSLContext` factory methods

[JDK-8372661]: https://bugs.openjdk.org/browse/JDK-8372661
[JDK-8373515]: https://bugs.openjdk.org/browse/JDK-8373515
[JDK-8373537]: https://bugs.openjdk.org/browse/JDK-8373537

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into simpleSslRest
 - Restore all non-`httpclient`, non-`httpserver` changes in `test/`
 - Make sure `get()` returns the same instance
 - Reverted all changes and only kept `SimpleSSLContext` enhancements
 - Overhaul `SimpleSSLContext` and its usages

Changes: https://git.openjdk.org/jdk/pull/28905/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28905&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373538
  Stats: 63 lines in 11 files changed: 0 ins; 39 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/28905.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28905/head:pull/28905

PR: https://git.openjdk.org/jdk/pull/28905