RFR: 7903785: Improve reporting for skipped tests

[Jonathan Gibbons]

This is to support the ability to track `skipped` tests in the JDK `make test` system.

It is somewhere between "proof of concept", "draft PR", and initial version of a real PR.

The primary goal is to post the number of `skipped` tests in a backwards compatible way that does not break existing makefile rules or require any changes.  The makefiles currently have some "fuzzy parsing" to search for named values that may appear on the `Test results:` line near the end of the `jtreg` output to the console.   This PR _adds_ a new value to that line _without changing any existing values_. In particular, the `passed` value is unchanged and continues to include the `skipped` tests, as before, but for readers and tools alike, the number of skipped tests is also reported.

The PR also exposes a previously undocumented feature, to control the formatting of that line, using a format string that can be given to `jtreg` in a system property. The set of format specifiers in that string has been extended to include format specifiers for "skipped tests" and "passed tests, excluding skipped tests".

In exposing that feature, some design questions come to mind:
* the terminology of "ignored" tests is somewhat confusing -- it is about tests that were not selected by virtue of the keyword filter (and related `-k` option)
* if the number of `skipped` tests is interesting, there are other similar numbers that might also be interesting, such as the number of tests that were filtered out by a `@requires` tag -- which is similar to but more powerful than the keyword filter. But these extra numbers are contained within a composite filter, and may not have been so easily accessible when `jtreg` was built using older versions of JDK -- that is, before covariant returns.

But to summarize, this feature is primarily about reporting the number of `skipped` tests. We can either defer supporting additional values until a subsequent PR, or put more work into this PR to expose more of these "interesting" numbers.

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Commit messages:
 - CODETOOLS-7903785: Improve reporting for skipped tests

Changes: https://git.openjdk.org/jtreg/pull/217/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=217&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903785
  Stats: 218 lines in 13 files changed: 203 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jtreg/pull/217.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/217/head:pull/217

PR: https://git.openjdk.org/jtreg/pull/217