[jdk11u-dev] RFR: 8272783: Epsilon: Refactor tests to improve performance

[Aleksey Shipilev]

Unclean backport to improve Epsilon testing. The original patch does not apply cleanly because the tests were moved to `gc.epsilon` package as part of bulk test move.

Additional testing: 
 - [x] Linux x86_64 fastdebug, `gc/epsilon`
 - [x] Linux x86_64 release, `gc/epsilon`

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Commit messages:
 - Backport 7f80683cfeee3c069f48d5bce45fa92b2381b518

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/453/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=453&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272783
  Stats: 407 lines in 21 files changed: 272 ins; 31 del; 104 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/453.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/453/head:pull/453

PR: https://git.openjdk.java.net/jdk11u-dev/pull/453