RFR: 8361715: (bf) Improve java/nio/Buffer/GetChars.java and migrate to JUnit [v6]

[Brett Okken]

> junit for CharSequence behavior of CharBuffer covering different CharBuffer implementations in various states (offset, position, limit)

Brett Okken has updated the pull request incrementally with one additional commit since the last revision:

  add randomized test params and null dst test case

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26282/files
  - new: https://git.openjdk.org/jdk/pull/26282/files/198e46e4..fe8c6043

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26282&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26282&range=04-05

  Stats: 66 lines in 1 file changed: 65 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26282.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26282/head:pull/26282

PR: https://git.openjdk.org/jdk/pull/26282