crypto: testmgr - Fix test disabling option

This patch fixes a serious bug in the test disabling patch where
it can cause an spurious load of the cryptomgr module even when
it's compiled in.

It also negates the test disabling option so that its absence
causes tests to be enabled.

The Kconfig option is also now behind EMBEDDED.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>