RFR: 8281511: java/net/ipv6tests/UdpTest.java fails with checkTime failed

[serhiysachkov]

switching to nanoTime as suggested in ticket comments

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Commit messages:
 - 8281511: java/net/ipv6tests/UdpTest.java fails with checkTime failed

Changes: https://git.openjdk.org/jdk/pull/23840/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23840&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8281511
  Stats: 9 lines in 1 file changed: 3 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/23840.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23840/head:pull/23840

PR: https://git.openjdk.org/jdk/pull/23840