Regression testing: a test should not run if a tool required by this test is missing

[ckhroulev]

Our installation manual suggests:

Run a basic suite of software tests. To do this, you need to be in PISM’s build directory, and the
cmake flag Pism_BUILD_EXTRA_EXECS should be ON. Then do:

$ make test

The message at the bottom should say “100% tests passed, 0 tests failed out of XX” or
similar. These tests use NCO and require Python packages numpy and netcdf4-python.

Few people read "these tests use NCO and require Python packages numpy and netcdf4-python" as "if you don't have NCO, numpy, and netcdf4-python some tests will fails and you should not be surprised".