RFR: 8116: build{.sh,.bat} fail to detect started p2 jetty process

[Patrick Reinhart]

This pull request fixes the detection of the started jetty p2 instance needed for the build to succeed.

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Commit messages:
 - 8116: Fix jetty startup detection

Changes: https://git.openjdk.org/jmc/pull/510/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=510&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8116
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jmc/pull/510.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/510/head:pull/510

PR: https://git.openjdk.org/jmc/pull/510