Remove unnecessary pragmas

We enable -Wall in singletons.cabal anyway, no need to enable
warn options explicitly in the source files.
Enable testsuite in .cabal file

Since #8180 is now fixed in HEAD the testsuite is enabled again.
Sadly, the insertion sort example does not build with HEAD.

I also added a set of targets in the Makefile to build the testsuite
with:

   make tests
Nuke trailing whitespaces in the testsuite