Add comment to tests/Makefile.
Add comment to tests/Makefile.

Add comment to tests/Makefile that users should use cabal to build and
run tests.

iu-parfunc/containers,Zigazou/containers,hvr/containers,shockkolate/containers,ekmett/containers,Zigazou/containers,iu-parfunc/containers,hvr/containers,iu-parfunc/containers,ekmett/containers,shockkolate/containers,hvr/containers,shockkolate/containers,Zigazou/containers,meiersi/psqueues-old,ekmett/containers