test: correct the test prefix in ut cmd_mem

Align the prefix used in cmd_ut_category function and name of tests
for ut mem.
This patch solves the issues detected by "make qcheck" after previous
patch.

Fixes: 550a9e79 ("cmd: Update the memory-search command")
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>

test/cmd/mem.c

@@ -15,6 +15,6 @@ int do_ut_mem(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 	struct unit_test *tests = ll_entry_start(struct unit_test, mem_test);
 	const int n_ents = ll_entry_count(struct unit_test, mem_test);
 
-	return cmd_ut_category("cmd_mem", "cmd_mem_", tests, n_ents, argc,
+	return cmd_ut_category("cmd_mem", "mem_test_", tests, n_ents, argc,
 			       argv);
 }