[PATCH v4] selftests: fix prepending $(OUTPUT) to $(TEST_PROGS)

[Ilya Leoshkevich <iii@linux.ibm.com>]

Currently the following command produces an error message:

    linux# make kselftest TARGETS=bpf O=/mnt/linux-build
    # selftests: bpf: test_libbpf.sh
    # ./test_libbpf.sh: line 23: ./test_libbpf_open: No such file or directory
    # test_libbpf: failed at file test_l4lb.o
    # selftests: test_libbpf [FAILED]

The error message might not affect make return code, therefore one might
need to grep make output in order to detect it.

The current logic prepends $(OUTPUT) only to the first member of
$(TEST_PROGS). After that, run_one() does

   cd `dirname $TEST`

For all tests except the first one, `dirname $TEST` is ., which means
they cannot access the files generated in $(OUTPUT).

Fix by using $(addprefix) to prepend $(OUTPUT)/ to each member of
$(TEST_PROGS).

Fixes: 1a940687e424 ("selftests: lib.mk: copy test scripts and test files for make O=dir run")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---

v1->v2:
- Append / to $(OUTPUT).
- Use $(addprefix) instead of $(foreach).

v2->v3:
- Split the patch in two.
- Improve the commit message.

v3->v4:
- Drop the first patch.
- Add a note regarding make return code to the commit message.

 tools/testing/selftests/lib.mk | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
index 1c8a1963d03f..0cf510df1ee2 100644
--- a/tools/testing/selftests/lib.mk
+++ b/tools/testing/selftests/lib.mk
@@ -75,7 +75,8 @@ ifdef building_out_of_srctree
 		@rsync -aq $(TEST_PROGS) $(TEST_PROGS_EXTENDED) $(TEST_FILES) $(OUTPUT)
 	fi
 	@if [ "X$(TEST_PROGS)" != "X" ]; then
-		$(call RUN_TESTS, $(TEST_GEN_PROGS) $(TEST_CUSTOM_PROGS) $(OUTPUT)/$(TEST_PROGS))
+		$(call RUN_TESTS, $(TEST_GEN_PROGS) $(TEST_CUSTOM_PROGS) \
+				  $(addprefix $(OUTPUT)/,$(TEST_PROGS)))
 	else
 		$(call RUN_TESTS, $(TEST_GEN_PROGS) $(TEST_CUSTOM_PROGS))
 	fi
-- 
2.23.0