In commit a5584b13d1e04f38b843602413669591aa65c359
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.
