crypto: testmgr - rename noextratests to noslowtests

Rename the noextratests module parameter to noslowtests, and replace
other remaining mentions of "extra" in the code with "slow".  This
addresses confusion regarding the word "extra" like that seen at
https://lore.kernel.org/r/6cecf2de-9aa0-f6ea-0c2d-8e974a1a820b@huawei.com/

.

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