RFR: 8340092: [Linux] containers/systemd/SystemdMemoryAwarenessTest.java failing on some systems

[Severin Gehwolf]

Please review this trivial test fix. The test fails if the `<slice>.d` directory already exists (for some reason). It needs to exist for the test to work. But if it exists we ignore the failure, thus using `Files.createDirectories()`.

As a benefit I've also added `exclusiveAccess.dirs=.` to the `systemd` test directory to match container tests in `docker` folder and added `TRUNCATE_EXISTING` for files in the `<slice>.d` directory. Both of which should ensure the test runs more reliably.

Testing:
- [ ] GHA
- [x] Manual testing by creating the directory first, running the test. Failed. The same handshake passes after the patch.

Thanks!

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Commit messages:
 - 8340092: [Linux] containers/systemd/SystemdMemoryAwarenessTest.java failing on some systems

Changes: https://git.openjdk.org/jdk/pull/20989/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20989&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340092
  Stats: 11 lines in 2 files changed: 7 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/20989.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20989/head:pull/20989

PR: https://git.openjdk.org/jdk/pull/20989