Tweak the compiler flags used in test suite.
Tweak the compiler flags used in test suite.

mrakgr/futhark,diku-dk/futhark,HIPERFIT/futhark,diku-dk/futhark,ihc/futhark,ihc/futhark,ihc/futhark,diku-dk/futhark,mrakgr/futhark,diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,mrakgr/futhark,HIPERFIT/futhark,CulpaBS/wbBach