RFR: 6944: build.sh does not correctly run UI tests due to not running p2 site

[Patrick Reinhart]

- calculate BASEDIR only once
- moved p2site (jetty) startup to separate method

Signed-off-by: Patrick Reinhart <patrick at reini.net>

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Commit messages:
 - 6944: moved p2site (jetty) startup to separate method

Changes: https://git.openjdk.java.net/jmc/pull/136/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=136&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6944
  Stats: 90 lines in 1 file changed: 46 ins; 42 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/136.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/136/head:pull/136

PR: https://git.openjdk.java.net/jmc/pull/136