RFR: JDK-8272318: Improve performance of HeapDumpAllTest

[Ralf Schmelter]

This change triggers a GC by System.gc() in the test case, so the created heap dump is significantly smaller and the test runs much faster.

-------------

Commit messages:
 - Improve performance of HeapDumpAllTest

Changes: https://git.openjdk.java.net/jdk/pull/5084/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5084&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272318
  Stats: 12 lines in 1 file changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5084.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5084/head:pull/5084

PR: https://git.openjdk.java.net/jdk/pull/5084