selftests/bpf: Print log buffer for exceptions test only on failure

selftests/bpf: Print log buffer for exceptions test only on failure

Alexei reported seeing log messages for some test cases even though we
just wanted to match the error string from the verifier. Move the
printing of the log buffer to a guarded condition so that we only print
it when we fail to match on the expected string in the log buffer,
preventing unneeded output when running the test.

Reported-by: Alexei Starovoitov <ast@kernel.org>
Fixes: d2a93715bfb0 ("selftests/bpf: Add tests for BPF exceptions")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230918155233.297024-2-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>