RFR: JDK-8329118: Run MessageFormat additional subformat pattern tests under en_US locale

[Justin Lu]

Please review this PR which updates two MessageFormat sub format related tests to be guaranteed to run under the `en_US` locale.

There exists locale that do not provide distinct instances for separate styles. For example, the `en_IN` locale provides the same LONG and SHORT compact number instances. The test data is built to test sub formats under the assumption that different styles do provide distinct instances.

As this is the case, these tests should be ran under a locale that does provide distinct instances for separate styles.

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Commit messages:
 - run in othervm
 - init

Changes: https://git.openjdk.org/jdk/pull/18498/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18498&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329118
  Stats: 8 lines in 2 files changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/18498.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18498/head:pull/18498

PR: https://git.openjdk.org/jdk/pull/18498