Don't set real log levels in tests

Setting the actual "log" "level" in tests triggers extraneous logging,
which is undesirable to have interleaved into test output.
diff --git a/test/config_tests.erl b/test/config_tests.erl
index d329bc9..50972d4 100644
--- a/test/config_tests.erl
+++ b/test/config_tests.erl
@@ -256,8 +256,8 @@
 should_update_option() ->
     ?_assertEqual("severe",
         begin
-            ok = config:set("log", "level", "severe", false),
-            config:get("log", "level")
+            ok = config:set("mock_log", "level", "severe", false),
+            config:get("mock_log", "level")
         end).
 
 should_create_new_section() ->
@@ -275,8 +275,8 @@
 should_return_undefined_atom_after_option_deletion() ->
     ?_assertEqual(undefined,
         begin
-            ok = config:delete("log", "level", false),
-            config:get("log", "level")
+            ok = config:delete("mock_log", "level", false),
+            config:get("mock_log", "level")
         end).
 
 should_be_ok_on_deleting_unknown_options() ->