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: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>