RFR: 7903953: Use scratch directory for JUnit's TempDir annotation

[Christian Stein]

Please review this change to use `jtreg`'s scratch directory for JUnit's `@TempDir` annotation.

Prior to this change, the standard implementation of JUnit Jupiter using the system default temporary directory is active, effectively decoupling the location of "interesting on failure" files from `jtreg`:
> When `jtreg` executes a test, the current directory for the test is set to a scratch directory so that the test can easily write any temporary files.

Find more details about `jtreg`'s scratch directory at https://openjdk.org/jtreg/faq.html#scratch-directory

In addition to use the current working directory for `@TempDir` annotated paths, this change also switches off JUnit's built-in cleanup of temporary files - leaving it to `jtreg` to care about it, see for example the help message for  the `-retain` command-line option:


    -retain | -retain:<pass,fail,error,all,file-pattern>,...
                    Specify files to be retained after each test completes
                    executing. If -retain is not specified, only the files from
                    the last test executed will be retained. If -retain is
                    specified with no argument, all files will be retained.
                    Otherwise, the files may be described by one or more of the
                    following values:
        none            Do not retain any of the files generated by each test
        pass            Retain files generated by tests that pass
        fail            Retain files generated by tests that fail
        error           Retain files generated by tests that caused an error
        all             Retain all files generated by each test
        file-pattern    Retain files that match a specific filename. The name
                        may contain '*' to match any sequence of characters. For
                        example, result.* or *.err.

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Commit messages:
 - 7903953: Use scratch directory for JUnit's TempDir annotation

Changes: https://git.openjdk.org/jtreg/pull/248/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=248&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903953
  Stats: 76 lines in 3 files changed: 74 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/248.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/248/head:pull/248

PR: https://git.openjdk.org/jtreg/pull/248