enhance FFTW easyblock to skip tests if --mpi-tests EasyBuild configuration option is disabled

[willfurnass]

Addresses #1100

Q: what's the idiomatic way to add a check for the toolchain having an MPI?

[ocaisa]

One way would be

if self.toolchain.mpi_family():

mpi_family() either returns the MPI family (OpenMPI/MPICH) or None is it is not defined

[willfurnass]

Thanks @ocaisa!

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS FFTW-3.3.9-gompi-2021a.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
node2603.swalot.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz (haswell), Python 2.7.5
See https://gist.github.com/5bf56cb4507308586622ed8937ad06a7 for a full test report.

[boegel]

lgtm