RFR: 8278131: runtime/cds/appcds/dynamicArchive/* tests failing in loom repo [v3]

[Calvin Cheung]

> A few dynamic archive CDS tests are failing in the loom repo due to more aggressive GC of code cache
> which leads to unloading of some classes before being written into a dynamic CDS archive.
> 
> This change is to make the affected tests more reliable by ensuring the test classes are not unloaded
> during dynamic CDS dump time.
> 
> Testing: Oracle CI tiers 1,2,4.

Calvin Cheung has updated the pull request incrementally with one additional commit since the last revision:

  add comments to helper classes

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6843/files
  - new: https://git.openjdk.java.net/jdk/pull/6843/files/e947fe06..736a5129

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6843&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6843&range=01-02

  Stats: 12 lines in 4 files changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6843.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6843/head:pull/6843

PR: https://git.openjdk.java.net/jdk/pull/6843