RFR: 8270094: Shenandoah: Provide human-readable labels for test configurations

[Aleksey Shipilev]

`hotspot_gc_shenandoah` contains a lot of parallel test configurations, and it would be much nicer to have them named appropriately, rather than just #id1...#id2.

Additional testing:
 - [x] `hotspot_gc_shenandoah`, eyeballing test names

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

Commit messages:
 - 8270094: Shenandoah: Provide human-readable labels for test configurations

Changes: https://git.openjdk.java.net/jdk/pull/4726/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4726&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270094
  Stats: 227 lines in 81 files changed: 19 ins; 0 del; 208 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4726.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4726/head:pull/4726

PR: https://git.openjdk.java.net/jdk/pull/4726