selftests/bpf: Move generated test files to $(TEST_GEN_FILES)

During an out-of-tree build, attempting to install the $(TEST_FILES)
into the $(OUTPUT) directory fails, because the objects were already
generated into $(OUTPUT):

rsync: [sender] link_stat "tools/testing/selftests/bpf/test_lwt_ip_encap.o" failed: No such file or directory (2)
rsync: [sender] link_stat "tools/testing/selftests/bpf/test_tc_edt.o" failed: No such file or directory (2)

Use $(TEST_GEN_FILES) instead.
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210113163319.1516382-4-jean-philippe@linaro.org

tools/testing/selftests/bpf/Makefile

@@ -44,10 +44,9 @@ ifneq ($(BPF_GCC),)
 TEST_GEN_PROGS += test_progs-bpf_gcc
 endif
 
-TEST_GEN_FILES =
-TEST_FILES = test_lwt_ip_encap.o \
-	test_tc_edt.o \
-	xsk_prereqs.sh
+TEST_GEN_FILES = test_lwt_ip_encap.o \
+	test_tc_edt.o
+TEST_FILES = xsk_prereqs.sh
 
 # Order correspond to 'make run_tests' order
 TEST_PROGS := test_kmod.sh \