Add max_nlev to more reg_test drivers

The stand-alone driver now reads in max_nlev from cvmix_nml (if it is not
included in the namelist, then max_nlev = nlev) and passes it to the three
tests that take nlev as input - Bryan-Lewis, double_diff, and shear-KPP. The
default for these three tests now allocate more memory than is needed for the
active ocean levels.