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

[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.

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Commit messages:
 - 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

Changes: https://git.openjdk.org/jdk/pull/28800/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28800&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373537
  Stats: 136 lines in 16 files changed: 52 ins; 24 del; 60 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