TestUtils: refuse to create temporary directory for non-Maven tests

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 just throw an Exception in
unknown cases. That way, we can add support for other test scenarios
and call those changes "bug fixes" rather than "backwards-incompatible
changes".

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>