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

[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

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Commit messages:
 - retry when a stray message is received
 - updated test to use Junit, also changed body of tests to be unqiue

Changes: https://git.openjdk.org/jdk/pull/12513/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12513&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301701
  Stats: 59 lines in 1 file changed: 17 ins; 20 del; 22 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