RFR: 8331342: Convert Base64 tests to JUnit [v2]

[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.

Justin Lu has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' into java.util.Base64-JUnit-conversions
 - Additional clarifying comments
 - convert TestBase64.java
 - convert TestBase64Golden.java
 - convert Base64GetEncoderTest.java

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19337/files
  - new: https://git.openjdk.org/jdk/pull/19337/files/945bc482..706293d4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19337&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19337&range=00-01

  Stats: 166493 lines in 2916 files changed: 110047 ins; 41299 del; 15147 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