RFR: CODETOOLS-7903371: Use multi-catch where applicable

[Jonathan Gibbons]

Please review a code-cleanup to use multi-catch where applicable, as well as some other cleanup suggested by an IDE.

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Commit messages:
 - CODETOOLS-7903371: Use multi-catch where applicable

Changes: https://git.openjdk.org/jtreg/pull/135/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=135&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903371
  Stats: 40 lines in 4 files changed: 1 ins; 13 del; 26 mod
  Patch: https://git.openjdk.org/jtreg/pull/135.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/135/head:pull/135

PR: https://git.openjdk.org/jtreg/pull/135