selftest/bpf: Extend the bpf_snprintf() test for "%c".

This patch adds various "positive" patterns for "%c" and two "negative"
patterns for wide character.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210814015718.42704-5-kuniyu@amazon.co.jp