In commit a5584b13
some scrubbing-related status variables were removed along with
the background scrubbing code.
The status variable INNODB_ENCRYPTION_NUM_KEY_REQUESTS
was inadvertently removed as part of that.

innodb_status_variables[]: Restore "encryption_num_key_requests".

We introduce the test innodb.innodb_status_variables
in order to catch similar regressions in the future.