test: correct the test prefix in ut str

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

Fixes: fdc79a6b ("lib: Add a function to convert a string to upper case")
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>