Use String::Random instead of File::Temp for just file prefixes

This allows us to drop TESTNAME completely and let File::Temp generate
and cleanup BASE directly. This also makes the cleanup method
obsolete.

For those tests which called cleanup directly inside a loop,
Test::UBH->new has to be called inside the loop so that the cleanup is
done automatically when the object instance goes out of scope.

Add String::Random as build-dependency to debian/control and
.travis.yml.