bpf/tests: Do not PASS tests without actually testing the result

Each test case can have a set of sub-tests, where each sub-test can
run the cBPF/eBPF test snippet with its own data_size and expected
result. Before, the end of the sub-test array was indicated by both
data_size and result being zero. However, most or all of the internal
eBPF tests has a data_size of zero already. When such a test also had
an expected value of zero, the test was never run but reported as
PASS anyway.

Now the test runner always runs the first sub-test, regardless of the
data_size and result values. The sub-test array zero-termination only
applies for any additional sub-tests.

There are other ways fix it of course, but this solution at least
removes the surprise of eBPF tests with a zero result always succeeding.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210721103822.3755111-1-johan.almbladh@anyfinetworks.com