selftests: kselftest_harness: generate test name once

Since we added variant support generating full test case
name takes 4 string arguments. We're about to need it
in another two places. Stop the duplication and print
once into a temporary buffer.

Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>