Move many-coll-test to ubuntu1804-wt-large and update thresholds accordingly

In order to be able to set ulimit n to a higher number, we need to run the many-coll-test on ubuntu1804-wt-large. This ticket requires the following work:

• Create a separate task so many-coll-test runs on ubuntu1804-wt-large. Keep many-dhandles-test on ubuntu1804-test.
• After running many-coll-test on the new variant, update the thresholds in the dedicated config in mongo-tests so the test passes sowe can start gathering data again.