prism-tests: unsupported CTL with step bounds test cases

Tweak the result strings for the CTL with step bounds cases (currently
computation is not supported with any engine). Previously, an error was
expected, but the properties are not actually erroneous. Additonally, the
error message was not uniform between the different engines, leading to
spurious failures against the exact engine.

prism-tests/functionality/verify/ctl/ctl.prism.props

@@ -104,13 +104,16 @@ A [ G E [ X E [X A [ G s=7 ] ] ] ]
 // RESULT: true
 A [ G E [ X E [X E [ X A [ G s=7 ] ] ] ] ]
 
-// RESULT: Error:bounded CTL operators is not supported
+
+// computation of bounded temporal operators for
+// CTL path formulas currently unsupported
+// RESULT: ?
 E [ F<3 A [ G s=7 ] ]
-// RESULT: Error:bounded CTL operators is not supported
+// RESULT: ?
 E [ G<3 s=0 ]
-// RESULT: Error:bounded CTL operators is not supported
+// RESULT: ?
 E [ s=0 U<3 s=1 ]
-// RESULT: Error:bounded CTL operators is not supported
+// RESULT: ?
 E [ s=0 R<3 s=1 ]
-// RESULT: Error:bounded CTL operators is not supported
+// RESULT: ?
 E [ s=0 W<3 s=1 ]