RFR: 8295153: java/util/Base64/TestEncodingDecodingLength.java ran out of memory [v3]

[Justin Lu]

> Please review this PR which converts _TestEncodingDecodingLength.java_ into a whitebox test which allows for testing to be done without memory usage issues.
> 
> Currently, the test requires about ~2.75 `Integer.MAX_VALUE` sized byte arrays worth of memory. (2 for the initial array allocation, .75 for the target array in `decode()`). While the `-Xms6g -Xmx8g` options should address this, there have been intermittent memory issues, as the underlying machine machine may be running other tests simultaneously.
> 
> By converting this test to a white-box test not only does it get rid of memory issues, but it also gets rid of the need to decode 2GB of data 3 times. The change is done using reflection to test the private visibility methods `encodedOutLength` and `decodedOutLength`, which the public `encode` and `decode` overloaded methods call respectively.

Justin Lu has updated the pull request incrementally with one additional commit since the last revision:

  reflect further review

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19036/files
  - new: https://git.openjdk.org/jdk/pull/19036/files/aa25c9a7..eb1f64ce

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19036&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19036&range=01-02

  Stats: 6 lines in 1 file changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19036.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19036/head:pull/19036

PR: https://git.openjdk.org/jdk/pull/19036