RFR: CODETOOLS-7903150: Restructure generated files for jtreg self-tests [v2]

[Jonathan Gibbons]

> Please review a mostly test-only fix to move all the output generated by the many jtreg self-tests into a new $(BUILDDIR)/test subdirectory, defined by a new BUILDTESTDIR variable.
> 
> All but two of the edited files are "just" test updates, and most of those were done with search/replace in the IDE. That gave a first pass, with about 90% success rate.  The remaining edits were done by fixing tests up manually, either because they had become broken, or because there were still writing files outside the $(BUILDTESTDIR) directory.
> 
> Of the two files that were edited that were not in the main `test` directory, one is in Defs.gmk, to define the new BUILDTESTDIR variable, and the other is in Makefile to add some opportunistic new clean targets.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  ensure $(BUILDTESTDIR) created

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

Changes:
  - all: https://git.openjdk.java.net/jtreg/pull/69/files
  - new: https://git.openjdk.java.net/jtreg/pull/69/files/1f3bc93e..8fa64e67

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtreg&pr=69&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jtreg&pr=69&range=00-01

  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/69.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/69/head:pull/69

PR: https://git.openjdk.java.net/jtreg/pull/69