RFR: 8332849: Update doc/testing.{md,html} (spelling and stale information)

[Mikael Vidstedt]

Update the testing doc to remove some stale information (AOT_MODULES, removed in [JDK-8264805](https://bugs.openjdk.org/browse/JDK-8264805)) and fix some spelling issues.

Testing: tier1

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Commit messages:
 - 8332849: Update doc/testing.{md,html} (spelling and stale information)

Changes: https://git.openjdk.org/jdk/pull/19375/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19375&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332849
  Stats: 46 lines in 2 files changed: 0 ins; 28 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/19375.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19375/head:pull/19375

PR: https://git.openjdk.org/jdk/pull/19375