[sw, dif_clkmgr] Add smoke test for clock manager DIF

Also replace 'sanity' with 'smoke' in the checklist as has already
been done for the other DIFs.

Signed-off-by: Michael Munday <mike.munday@lowrisc.org>
diff --git a/test/systemtest/config.py b/test/systemtest/config.py
index 065a8a4..8b03a84 100644
--- a/test/systemtest/config.py
+++ b/test/systemtest/config.py
@@ -60,6 +60,9 @@
         "name": "dif_uart_smoketest",
     },
     {
+        "name": "dif_clkmgr_smoketest",
+    },
+    {
         "name": "flash_ctrl_test",
     },
     {