Re #7521. Speed up test 3x by using a smaller IDF.
Re #7521. Remove check that accounts for 60% of test time.

I think we can manage without this as using MERLIN is tested
in a system test.
Re #7521. Speed up test.

This code isn't used at the SNS anyway (we use DGSReduction).
Merge branch 'bugfix/7521_slow_unit_tests' into develop