tests: Trim .nim files trailing whitespace

via OSX: find . -name '*.nim' -exec sed -i '' -E 's/[[:space:]]+$//' {} +

tests/controlflow/tstatret.nim

@@ -3,10 +3,10 @@ discard """
   line: 9
   errormsg: "statement not allowed after"
 """
-# no statement after return
-proc main() =
-  return
-  echo("huch?") #ERROR_MSG statement not allowed after
-
+# no statement after return
+proc main() =
+  return
+  echo("huch?") #ERROR_MSG statement not allowed after
+