RFR: 8372661: Add a null-safe static factory method to "jdk.test.lib.net.SimpleSSLContext" [v3]

[Volkan Yazici]

> Overhauls `SimpleSSLContext` to remove the need for null checks at the call site, and to accept a key store file search path, which removes the need to copy-paste `SimpleSSLContext` just to change the search path.

Volkan Yazici has updated the pull request incrementally with one additional commit since the last revision:

  Make sure `get()` returns the same instance

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28765/files
  - new: https://git.openjdk.org/jdk/pull/28765/files/d4c9b312..b52e442f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28765&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28765&range=01-02

  Stats: 23 lines in 1 file changed: 10 ins; 7 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/28765.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28765/head:pull/28765

PR: https://git.openjdk.org/jdk/pull/28765