crypto: testmgr - replace CRYPTO_MANAGER_DISABLE_TESTS with CRYPTO_SELFTESTS

The negative-sense of CRYPTO_MANAGER_DISABLE_TESTS is a longstanding
mistake that regularly causes confusion.  Especially bad is that you can
have CRYPTO=n && CRYPTO_MANAGER_DISABLE_TESTS=n, which is ambiguous.

Replace CRYPTO_MANAGER_DISABLE_TESTS with CRYPTO_SELFTESTS which has the
expected behavior.

The tests continue to be disabled by default.

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