RFR: 8331342: Convert Base64 tests to JUnit

[Justin Lu]

Please review this test-only clean up PR which converts the java.util.Base64 tests to run under JUnit.

In general, this allows for the tests to run independently, separates the data providers from the tests, as well being able to utilize the built in JUnit utility test methods to reduce boilerplate testing code.

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Commit messages:
 - Additional clarifying comments
 - convert TestBase64.java
 - convert TestBase64Golden.java
 - convert Base64GetEncoderTest.java

Changes: https://git.openjdk.org/jdk/pull/19337/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19337&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331342
  Stats: 823 lines in 3 files changed: 257 ins; 336 del; 230 mod
  Patch: https://git.openjdk.org/jdk/pull/19337.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19337/head:pull/19337

PR: https://git.openjdk.org/jdk/pull/19337