Heads up: top level `make test` now runs the full testsuite

[Thomas Miedema]

Typing `make test` in the ghc root directory now runs the full
testsuite, just like it already did in the `testsuite/tests`
directory.

If you want to run the fast testsuite, use the new targets `make
fasttest` or `make fast`.

The `validate` script still runs the same tests as before (depending
on the argument, `--slow`, `--fast` or normal).

References:
https://ghc.haskell.org/trac/ghc/wiki/Building/StandardTargets#Topleveltargets
https://ghc.haskell.org/trac/ghc/wiki/Building/RunningTests

-Thomas Miedema