RFR: 7903738 : jtr.xml logs produced with -xml argument do not contain compilation failure justification - alternative approach

[andrlos]

this is an alternative to https://github.com/openjdk/jtreg/pull/198
here we are collecting all the System.err outputs and putting them into a StringBuilder and returning them as one long string..
Order of the sections should be intact, so it should go chronologically according to the test execution.
This could be also improved by adding section separator Strings into the StringBuilder between individual sections.
Also we could drop the `if (titles[i].equals("main") || ...` part and output any System.err output found in jtr.xml as part of the test failure

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Commit messages:
 - alternative approach to jtr.xml transcription

Changes: https://git.openjdk.org/jtreg/pull/199/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=199&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903738
  Stats: 5 lines in 1 file changed: 1 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jtreg/pull/199.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/199/head:pull/199

PR: https://git.openjdk.org/jtreg/pull/199