TestUtils: never create test directories in /tmp/

Curtis Rueden offered reluctance to fall back to creating tests in
/tmp/ when it was determined that the tests are run in in unknown
location.

This developer agrees. Therefore, let's test carefully that we found
some sensible location close to the .class files, handling unit tests
run via Maven extra nicely.

For details, see the pull request corresponding to this topic branch:
https://github.com/scijava/scijava-common/pull/70

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Be careful in createTemporaryDirectory() when removing existing files

If File.exists() returns true, it might be a regular file being in
the way, after all...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>