hi,
Are you sure you have the proper kernel code loaded with the test firmware code that creates this sysfs file? Without that, this test will not work.
I checked following config options, and they all enabled.
tools/testing/selftests/firmware/config CONFIG_TEST_FIRMWARE=y CONFIG_FW_LOADER=y CONFIG_FW_LOADER_USER_HELPER=y CONFIG_IKCONFIG=y CONFIG_IKCONFIG_PROC=y CONFIG_FW_UPLOAD=y
best regards,