Use os.devnull instead of '/dev/null' in the testsuite driver.
Use os.devnull instead of '/dev/null' in the testsuite driver.

The later caused issues on windows by being translated into
"\\dev\\null" and python then trying to open this non-existant file.

So we now use os.devnull inside python and convert it to "/dev/null"
when calling out to the shell, which is bound to run in a unix like
environment.

This fixes an issue a test producing unexpected stderr output failed
with a framework failure instead of showing a diff of the output.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc