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

[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.

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Commit messages:
 - CODETOOLS-7903150: Restructure generated files for jtreg self-tests

Changes: https://git.openjdk.java.net/jtreg/pull/69/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=69&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903150
  Stats: 824 lines in 142 files changed: 16 ins; 0 del; 808 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