RFR: 8353738: Update TLS unit tests to not use certificates with MD5 signatures [v4]

[Matthew Donovan]

> This PR updates tests that were using MD5 certificates. For most of the tests, I added test cases for TLSv1.2/MD5withRSA and TLSv1.3/SHA256withRSA.

Matthew Donovan has updated the pull request incrementally with one additional commit since the last revision:

  removed unnecessary comment and made getSSLContext(...) private

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27342/files
  - new: https://git.openjdk.org/jdk/pull/27342/files/4804c90d..94ea9ba7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27342&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27342&range=02-03

  Stats: 22 lines in 6 files changed: 0 ins; 13 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/27342.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27342/head:pull/27342

PR: https://git.openjdk.org/jdk/pull/27342