RFR: 8320750: Allow a testcase to run with multiple -Xlog [v3]

[Leo Korinth]

> Running a testcase with muliple -Xlog crashes JTREG test cases. This is because `Collector.toMap` is not given a merge strategy.
> 
> When the same argument is passed multiple times, I have added a merge strategy to use the latter value. This is similar to how it is implemented for `vm.opt.*` in JTREG.
> 
> If the flag tested is `-Xlog`, replace the value part with a dummy value "NONEMPTY_TEST_SENTINEL". This is because in the case of multiple `-Xlog` all values are used, and JTREG does not give a satisfactory way to represent them. This dummy value should make it hard to try to `@require` on specific values by mistake.
> 
> Tested with:
> 
>  @requires vm.opt.x.Xlog == "NONEMPTY_TEST_SENTINEL"
>  @requires vm.opt.x.Xlog == "NONEMPTY_TEST_SENTINELXXX"
>  @requires vm.opt.x.Xms == "3g"
> 
> and
> 
> JAVA_OPTIONS=-Xms3g -Xms4g
> JAVA_OPTIONS=-Xms4g -Xms3g
> JAVA_OPTIONS=-Xlog:gc* -Xlog:gc*
> ``` 
> 
> Running tier1

Leo Korinth has updated the pull request incrementally with one additional commit since the last revision:

  fix after comments from David

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16824/files
  - new: https://git.openjdk.org/jdk/pull/16824/files/d129a0fb..53a3bc2c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16824&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16824&range=01-02

  Stats: 11 lines in 2 files changed: 7 ins; 1 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16824.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16824/head:pull/16824

PR: https://git.openjdk.org/jdk/pull/16824