test: inconsistent bootm tests

Excluding ut bootm in test/cmd_ut.c but compiling test/bootm.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_bootm_nop]
FAILED test/py/tests/test_ut.py::test_ut[ut_bootm_nospace]
FAILED test/py/tests/test_ut.py::test_ut[ut_bootm_silent]
FAILED test/py/tests/test_ut.py::test_ut[ut_bootm_silent_var]
FAILED test/py/tests/test_ut.py::test_ut[ut_bootm_subst]
FAILED test/py/tests/test_ut.py::test_ut[ut_bootm_subst_both]
FAILED test/py/tests/test_ut.py::test_ut[ut_bootm_subst_var]

Only compile test/bootm.c on the sandbox.

Fixes: f158ba15 ("bootm: Add tests for fixup_silent_linux()")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>