RFR: 8322971: KEM.getInstance() should check if a 3rd-party security provider is signed

[Weijun Wang]

`KEM.getInstance` now checks if the implementation is from a signed provider if it's not builtin to JDK.

Several adjustments to the test:
1. Put one impl in `SunEC` to pretend it's builtin. This is necessary to test for provider selection.
2. When there is no need to choose a provider, use reflection to create a `KEM` object that bypasses the `getInstance` call.

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Commit messages:
 - the fix

Changes: https://git.openjdk.org/jdk/pull/17253/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17253&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322971
  Stats: 119 lines in 4 files changed: 74 ins; 29 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/17253.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17253/head:pull/17253

PR: https://git.openjdk.org/jdk/pull/17253