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.