Better cleanup in java/nio/file/Files/probeContentType/ParallelProbes.java

ParallelProbes.java creates a prefix*.json file in the default temp directory, but does not clean it up later.

cat -n jdk/test/java/nio/file/Files/probeContentType/ParallelProbes.java
   [...]
    50 final Path p = Files.createTempFile("prefix", ".json");

A better approach would be to create temporary test files under the jtreg property user.dir instead.

During a jtreg run, user.dir has the value $PWD/JTwork/scratch