test_firmware: Install all scripts

List all the scripts invoked by fw_run_tests.sh, so that
"make TARGETS=firmware install" keeps working.

Fixes: 29a1c00ce1df8 ("test_firmware: add simple firmware firmware test ...")
Fixes: b3cf21fae1fe0 ("test_firmware: test three firmware kernel configs ...")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/testing/selftests/firmware/Makefile

@@ -4,6 +4,7 @@
 all:
 
 TEST_PROGS := fw_run_tests.sh
+TEST_FILES := fw_fallback.sh fw_filesystem.sh fw_lib.sh
 
 include ../lib.mk