RFR: 6734346: harness reads too many files

[Daniel Jeliński]

This patch fixes a long-standing issue where jtreg reads every java file under a test root (like test/jdk) when generating reports for jobs running a subset of tests.
On a Windows machine, this reduces the time required to run a single (fast) test and generate report from 15-20 seconds to 1-3 seconds.

Verified that the generated report looks correct; the report only contains tests that were actually run, so the extra work in scanning every java file was unnecessary.

Did not verify if the existing tests still pass; I'm new to ant, and couldn't get it to run the tests. Are there any instructions I could use?

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Commit messages:
 - Avoid scanning the entire test root during report generation

Changes: https://git.openjdk.org/jtharness/pull/38/files
 Webrev: https://webrevs.openjdk.org/?repo=jtharness&pr=38&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-6734346
  Stats: 50 lines in 5 files changed: 19 ins; 27 del; 4 mod
  Patch: https://git.openjdk.org/jtharness/pull/38.diff
  Fetch: git fetch https://git.openjdk.org/jtharness pull/38/head:pull/38

PR: https://git.openjdk.org/jtharness/pull/38