Bug fix: randomize createTemporaryDirectory(String, Class) again

Callers of the createTemporaryDirectory(String, Class) method (I am
looking at you, imagej-updater!) started to expect multiple calls to
return different values. So let's pseudo-randomize: have a global
counter...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make sure that TestUtils' createTemporaryDirectory works correctly

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Help certain callers of the TestUtils class

In certain components, it is very convenient to have utility methods for
testing data structures involving temporary directories. For example,
the updater tests want to create a temporary directory for the
ImageJ.app/ directory, another one for the update site, and then bundle
them into a FilesCollection data structure. Such methods are typically
stored in classes with the suffix ...TestUtils, so let's special-case
those classes and ignore them when trying to figure out the call site.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'fix-test-utils'

The goodies ported from MiniMaven's TestUtils class actually had a bug,
now unfortunately in the scijava-common version 2.20.0. This bug was
identified when trying to run imagej-updater's test suite against said
version, and have been fixed in this topic branch.

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