Fix warning text when a test case is missing

The warning message shown when a test case is missing was garbled. Fix
the message so it correctly states that an expected test is not present.

Change-Id: I3202572b57dd47b6925aad7f4d28d586c5a4a677
BUG: 791125
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522154Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Eric Lawrence [MSFT] <ericlaw@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#824875}

components/policy/core/browser/policy_pref_mapping_test.cc

@@ -400,7 +400,7 @@ void VerifyAllPoliciesHaveATestCase(const base::FilePath& test_case_path) {
     LOG_IF(WARNING, !has_test_case_for_this_os)
         << "Policy " << policy->first
         << " is marked as supported on this OS in policy_templates.json but "
-        << "have a test for this platform in policy_test_cases.json.";
+        << "there is no test for this platform in policy_test_cases.json.";
   }
 }