Fix the comparison in PetscJacobianTester (closes #3912).

It was missing the type cast, so the result was wrong even if the
numbers did not satisfy the relation.
Adjusting tolerances in Richards module (refs #3912)

Four tests have a prescribed tolerance smaller that it was actually
computed by the test. After a fix in PetscJacobianTester, these started
to fail (that was expected). Using a looser tolerance instead.
Merge pull request #3913 from andrsd/fix-petsc-jacobian-tester

Fix the comparison in PetscJacobianTester (closes #3912).