RFR: CODETOOLS-7903165: change -encoding option to use ASCII instead of ISO8859

[Jonathan Gibbons]

Please review a trivial change to replace use of `-encoding ISO8859-1` (in various forms) to `-encoding ASCII`

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Commit messages:
 - CODETOOLS-7903165: change -encoding option to use ASCII instead of ISO8859

Changes: https://git.openjdk.java.net/jtreg/pull/76/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=76&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903165
  Stats: 12 lines in 8 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/76.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/76/head:pull/76

PR: https://git.openjdk.java.net/jtreg/pull/76