add tests for Testament "reject" action (#15709)

* add test for reject action

* highlight that a compilation failure is expected

* add the converse test for action=reject

* add trailing newline

* fix wording

testament/tests/shouldfail/treject.nim

@@ -0,0 +1,7 @@
+discard """
+action: "reject"
+"""
+
+# Because we set action="reject", we expect this line not to compile. But the
+# line does compile, therefore the test fails.
+assert true