test/config: adjust initial permissions in a test

The test gives some initial permissions for a newly created user. Then,
it synchronizes the permissions with ones that are given in the new
configuration format. The resulting permissions should be the same as
described in the configuration (plus defaults).

The code that grants the initial permissions has a typo in one of the
permission descriptions, so it is not granted (and this error is
silently ignored). It doesn't affect the test case successfulness,
because this permission is anyway expected to be revoked by the
synchronization.

The patch fixes the initial privilege granting code and makes the test
actually verifying that the given permission is revoked.

Part of #8967

NO_DOC=It is a fix of a test.
NO_CHANGELOG=see NO_DOC

test/config-luatest/credentials_applier_test.lua

@@ -430,7 +430,7 @@ g.test_sync_privileges = function(g)
         }, {
             "revoke", "session,usage", "universe", ""
         }, {
-            "grant", "execute", "functions", ""
+            "grant", "execute", "function", ""
         },
     }