Make spherical_shell tests more robust ref #3633

These tests had very small loads applied, which resulted in tiny
displacements, so they couldn't converge to very tight relative
tolerances.  Multiplied the load by 1000 and tightened the tolerance.
Also removed the explicit setting of pc_hypre_boomeramg_strong_threshold.
Merge pull request #3715 from bwspenc/fix_tests

Make spherical_shell tests more robust