RFR: 8270341: Test serviceability/dcmd/gc/HeapDumpAllTest.java timed-out

[Chris Plummer]

[JDK-8267666](https://bugs.openjdk.java.net/browse/JDK-8267666) added a 2nd test case, which doubles the run time of this test. As a result it occasionally times out. This change double the timeout to 240 seconds.

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Commit messages:
 - Longer timeout for HeapDumpAllTest needed after new test case added.

Changes: https://git.openjdk.java.net/jdk/pull/4935/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4935&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270341
  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4935.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4935/head:pull/4935

PR: https://git.openjdk.java.net/jdk/pull/4935