test: inconsistent string tests

Excluding ut str in test/cmd_ut.c but compiling test/str_ut.c results in
failure of the Python test invoking the C unit tests as observed on
sipeed_riscv_smode_defconfig:

FAILED test/py/tests/test_ut.py::test_ut[ut_str_upper]

Allow to compile test/str_ut.c on all boards.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>