doc: update information on test/known_issues

* Since #6559 known_issues
  does run on CI.
* Add some notes to explain the expectations around tests in
  known_issues.

PR-URL: #12262
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: Jan Krems

test/README.md

@@ -90,8 +90,12 @@ On how to run tests in this direcotry, see
     </tr>
     <tr>
       <td>known_issues</td>
-      <td>No</td>
-      <td>Tests reproducing known issues within the system.</td>
+      <td>Yes</td>
+      <td>
+        Tests reproducing known issues within the system. All tests inside of
+        this directory are expected to fail consistently. If a test doesn't fail
+        on certain platforms, those should be skipped via `known_issues.status`.
+      </td>
     </tr>
     <tr>
       <td>message</td>