selftests/bpf: fix clearing buffered output between tests/subtests

Clear buffered output once test or subtests finishes even if test was
successful. Not doing this leads to accumulation of output from previous
tests and on first failed tests lots of irrelevant output will be
dumped, greatly confusing things.

v1->v2: fix Fixes tag, add more context to patch

Fixes: 3a516a0a ("selftests/bpf: add sub-tests support for test_progs")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>