testsuite: Testing ./Setup interface for failure mode

[mpickering]

Currently we have code in the setup'' helper which says:

 189             else do                                                             
 190               r <- liftIO $ runghc (testScriptEnv env)                          
 191                                    (Just (testCurrentDir env))                  
 192                                    (testEnvironment env)                        
 193                                    (testCurrentDir env </> prefix </> "Setup.hs")
 194                                    (NE.toList full_args)                        
 195               recordLog r  
 196               requireSuccess r

The result of calling requireSuccess means that the call to the Setup script above has to succeed, but what about if we want to check the failure modes of ./Setup? In particular I want to check that it doesn't accept external commands (see #9403)

Am I missing way to test the interface of custom ./Setup scripts?

Proposal: Don't call requireSuccess r here, and require users of setup'' and setup' to check the exit code themselves.

This essentially reverts the commit 326da71, because it isn't what I wanted and not necessarily what you want if you want to test that ./Setup fails in a certain way.

[mpickering]

I now read the source of requireSuccess and infact it seems that I need to use the fails modifier to indicate that the call will fail, then we check the exit code is not ExitSuccess.