Revert "remove CMakeCache.txt before running the test"

This reverts commit df3dc678 which has the comment:

This is needed only if we are using custom conda environments instead of those from cvmfs. In fact, if we re-configure, the cmake got confused by the mismatch of paths (e.g. to ninja executable living in conda environment bin directory used by the build task (which is not available for the test task)).