RFR: 8370333: hotspot-unit-tests.md specifies wrong directory structure for tests

[Kim Barrett]

Please review this change to the HotSpot unit test documentation, fixing the
path where native tests are located.

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Commit messages:
 - fix path for native test location

Changes: https://git.openjdk.org/jdk/pull/28251/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28251&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370333
  Stats: 8 lines in 2 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/28251.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28251/head:pull/28251

PR: https://git.openjdk.org/jdk/pull/28251