[jdk18u] RFR: 8282042: [testbug] FileEncodingTest.java depends on default encoding

[Tyler Steele]

Backporting this small testbug fix to jdk18u as requested [here](https://github.com/eclipse-openj9/openj9/issues/14472#issuecomment-1048328921)

This issue fixes a testbug present on AIX.

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Commit messages:
 - Backport 58e1882f3ccc648c5f6d216d37cfd1805889b8d8

Changes: https://git.openjdk.java.net/jdk18u/pull/96/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk18u&pr=96&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282042
  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk18u/pull/96.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18u pull/96/head:pull/96

PR: https://git.openjdk.java.net/jdk18u/pull/96