RFR: 8258512: serviceability/sa/TestJmapCore.java timed out on macOS 10.13.6

[Chris Plummer]

Increase the test timeout to 480 just to make sure we don't see this timeout anymore. This same change was already made to TestJmapCoreMetaspace.java a long time ago.

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Commit messages:
 - Increase test timeout from 240 to 480

Changes: https://git.openjdk.java.net/jdk/pull/6676/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6676&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258512
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6676.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6676/head:pull/6676

PR: https://git.openjdk.java.net/jdk/pull/6676