RFR: 8301701 : java/net/DatagramSocket/DatagramSocketMulticasting.java should be hardened [v6]

[Darragh Clarke]

> Updated `DatagramSocketMulticasting` to use Junit and also hardened the test to avoid occasional interference from other tests.
> 
> - Test now uses Junit Assertions, in general if the assertion used had been `assertTrue(foo==bar)` I replaced it with `assertEquals(foo,bar)` though there are some cases where I used `assertTrue` or `assertFalse`
> - `testSendReceive` now retries up to 3 times if a message from an unexpected port is received
> - Both `testSendReceive` and `testSendNoReceive` both have messages that use the methods name instead of just "hello"
> 
> Ran tiers 1-3 of tests as well as running this updated test to make sure it was stable

Darragh Clarke has updated the pull request incrementally with one additional commit since the last revision:

  include charset to be used when new string is created

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/12513/files
  - new: https://git.openjdk.org/jdk/pull/12513/files/08428a32..1511fc10

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12513&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12513&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12513.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12513/head:pull/12513

PR: https://git.openjdk.org/jdk/pull/12513