crypto_test: add a mutex to serialize prints
crypto_test: add a mutex to serialize prints

The concurrency test output is garbled and difficult
to read when the lines don't get printed completely.
Avoid this by using a mutex.

Signed-off-by: Naveen Kaje <79e427f88b1293e6ef90566b3bd0a85565336125@juul.com>

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core