Modify unit tests so that any files created are rooted in the directory
specified by the "tmp.dir" Ant property. Ant already passes the value of
tmp.dir to the JVM's java.io.tmpdir parameter, so the primary changes are
to make use of Files.createTempDirectory to generate per-test work
directories. Also update the test checkstyle rules to catch any future
usage of hardcoded "/tmp".
Patch by Derek Chen-Becker; reviewed by bereng and brandonwilliams for CASSANDRA-18750
Maxim Muzafarov