testament: error instead of silently overwrite a spec (#16166)

tests/casestmt/tcaseexpr1.nim

@@ -1,17 +1,23 @@
 discard """
-  errormsg: "type mismatch: got <string> but expected 'int'"
-  line: 33
-  file: "tcaseexpr1.nim"
-
-  errormsg: "not all cases are covered; missing: {C}"
-  line: 27
-  file: "tcaseexpr1.nim"
+  cmd: "nim check $options $file"
+  action: "reject"
+  nimout: '''
+tcaseexpr1.nim(33, 10) Error: not all cases are covered; missing: {C}
+'''
 """
 
-# NOTE: This spec is wrong. Spec doesn't support multiple error
-# messages. The first one is simply overridden by the second one.
-# This just has never been noticed.
+#[
+# xxx make nimout comparison use nimoutCheck instead of:
+elif expected.nimout.len > 0 and expected.nimout.normalizeMsg notin given.nimout.normalizeMsg:
 
+and then use nimout: '''
+tcaseexpr1.nim(33, 10) Error: not all cases are covered2; missing: {C}
+tcaseexpr1.nim(39, 12) Error: type mismatch: got <string> but expected 'int literal(10)'
+'''
+]#
+
+
+# line 20
 type
   E = enum A, B, C