crypto: api - compile out crypto_boot_test_finished when tests disabled

The crypto_boot_test_finished static key is unnecessary when self-tests
are disabled in the kconfig, so optimize it out accordingly, along with
the entirety of crypto_start_tests().  This mainly avoids the overhead
of an unnecessary static_branch_enable() on every boot.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>