RFR: 7903601 : simple fix for jtr.xml logs missing description with shell tests

[andrlos]

issue:
https://bugs.openjdk.org/browse/CODETOOLS-7903601

with "-xml" argument when a shell driven test fails, the only thing you are getting in jtr.xml file as an explanation is the exit code which is not very helpful.. this is a fix for that which aligns the behavior of shell tests with that of java tests. The solution is up for a discussion, I just did the simplest and sturdiest solution I could think of.. in case there is no "main" section in the original jtr file, the xmlwriter in jtreg tries to iterate the sections again for "shell" section and writes the failure it finds there to the jtr.xml log

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Commit messages:
 - simple fix for missing shell failures in jtr.xml logs that has been in jtr logs

Changes: https://git.openjdk.org/jtreg/pull/175/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=175&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903601
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jtreg/pull/175.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/175/head:pull/175

PR: https://git.openjdk.org/jtreg/pull/175