RFR: 7904107: AgentOut.othervm.ok should not access $(BUILDTESTDIR)/AgentOut.agentvm

[Vladimir Petko]

AgentOut.othervm.ok tries to access $(BUILDTESTDIR)/AgentOut.agentvm. This should only happen in AgentOut.agentvm.ok. 
Add a new target to filter log lines that depends on AgentOut.agentvm.ok.

Note: tests pass on retry[1]. The test failure is caused by [2]

[1] https://github.com/vpa1977/jtreg/actions/runs/18863066182
[2] https://github.com/openjdk/jtreg/pull/297

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Commit messages:
 - fix: filter agent output after agent target

Changes: https://git.openjdk.org/jtreg/pull/299/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=299&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904107
  Stats: 8 lines in 1 file changed: 4 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/299.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/299/head:pull/299

PR: https://git.openjdk.org/jtreg/pull/299