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

tests/testament/treject.nim

@@ -0,0 +1,6 @@
+discard """
+action: "reject"
+"""
+
+# this line does not compile, so the test should pass, since action="reject"
+let x: int = "type mismatch"

tests/testament/tshould_not_work.nim

@@ -25,6 +25,8 @@ FAIL: tests/shouldfail/toutput.nim C
 Failure: reOutputsDiffer
 FAIL: tests/shouldfail/toutputsub.nim C
 Failure: reOutputsDiffer
+FAIL: tests/shouldfail/treject.nim C
+Failure: reFilesDiffer
 FAIL: tests/shouldfail/tsortoutput.nim C
 Failure: reOutputsDiffer
 FAIL: tests/shouldfail/ttimeout.nim C