Inject arbitrary fails into xcosi

src/xcosi/test

 xcase__test() {
     jat__cmd test -s Foo
     jat__cmd test -s Bar
+    test "$Foo:$Bar" == "f1:b2" && jat__cmd false
+    test "$Foo:$Bar" == "f0:b0" && jat__log_warning heya
+    test "$Foo:$Bar" == "f1:b1" && jat__log_error kaboom
 }
 
 xcase__run -v