RFR: CODETOOLS-7902983: jcstress: Improve performance for tests with many results

[Aleksey Shipilev]

As reported here:
  https://mail.openjdk.java.net/pipermail/jcstress-dev/2021-June/000702.html

Cache the grading, optimize HTML table generation a little.

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

Commit messages:
 - CODETOOLS-7902983: jcstress: Improve performance for tests with many results

Changes: https://git.openjdk.java.net/jcstress/pull/86/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcstress&pr=86&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902983
  Stats: 14 lines in 2 files changed: 13 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jcstress/pull/86.diff
  Fetch: git fetch https://git.openjdk.java.net/jcstress pull/86/head:pull/86

PR: https://git.openjdk.java.net/jcstress/pull/86