[build] Put the brackets in the right place this time

Now we will properly fail when the exit code is non-zero. I really
hate make.
[mdtool] Fail if a test using GuiUnit cannot load GuiUnit.exe

We should not hide this error, we should fail instantly. If we hid
this error we would end up running the tests with the wrong test
runner which would give us strange and unusual (and useless) output.