fix regression: -d:nimHasLibFFI was not being tested anymore (#14234)

* * fix regression: -d:nimHasLibFFI was not being tested anymore,
in part because testament was silently treating some errors as easy to overlook messages
* turned that message into an error
* -d:nimHasLibFFI is now being tested with nim cpp
* use correct signatures for importc procs
* workaround for openbsd to unblock ctffi testing

testament/categories.nim

@@ -545,7 +545,7 @@ proc processSingleTest(r: var TResults, cat: Category, options, test: string) =
   if existsFile(test):
     testSpec r, makeTest(test, options, cat), {target}
   else:
-    echo "[Warning] - ", test, " test does not exist"
+    doAssert false, test & " test does not exist"
 
 proc isJoinableSpec(spec: TSpec): bool =
   result = not spec.sortoutput and