Tests fail on more than 2 procs

[lukasdreyer]

The following tests fail when run on more than 2 procs:

test/t8_cmesh/t8_gtest_multiple_attributes (first fails for 3 procs, Segmentation fault)

test/t8_forest_incomplete/t8_gtest_empty_local_tree (not failing, but running way too slow for multiple procs)

test/t8_forest_incomplete/t8_gtest_iterate_replace (first fails for 5 procs, t8_forest_partition.cxx:245):
T8_ASSERT (last_desc_id < first_desc_id);

(More fail before merging #1086)

[holke]

#1100 may fix test/t8_forest_incomplete/t8_gtest_iterate_replace, please check

[holke]

#1142 should resolve the test/t8_forest_incomplete/t8_gtest_iterate_replace

[Davknapp]

Our test-suite tests now with 4 procs. Furthermore one can add a custom command to use even more that 4 procs to test in parallel.