add an option to turn on slow tests?

[edwardhartnett]

On many of my packages (like netcdf-c) I include an extra option to turn on slow tests, by default this option is off. In CI runs I am careful to turn it on if possible.

The reason is that users will stop running tests if they take too long. I'm running the ip tests now and they are taking a long time. If the three or four longest tests we only run when -DENABLE_SLOW_TESTS=ON, that might be better.

For example:

 22/110 Test  #22: test_rotatedB_spectral_vector_grib2_4 ....................   Passed  188.35 sec
        Start  23: test_rotatedE_budget_vector_grib2_4