selftests: Fix build when $(O) points to a relative path

Build of bpf and tc-testing selftests fails when the relative path of
the build directory is specified.

make -C tools/testing/selftests O=build0
make[1]: Entering directory '/linux_mainline/tools/testing/selftests/bpf'
../../../scripts/Makefile.include:4: *** O=build0 does not exist.  Stop.
make[1]: Entering directory '/linux_mainline/tools/testing/selftests/tc-testing'
../../../scripts/Makefile.include:4: *** O=build0 does not exist.  Stop.

Makefiles of bpf and tc-testing include scripts/Makefile.include file.
This file has sanity checking inside it which checks the output path.
The output path is not relative to the bpf or tc-testing. The sanity
check fails. Expand the output path to get rid of this error. The fix is
the same as mentioned in commit 150a2732 ("bpf, preload: Fix build
when $(O) points to a relative path").

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>