INNODB_ENCRYPTION_NUM_KEY_REQUESTS is missing from INFORMATION_SCHEMA.GLOBAL_STATUS

In MDEV-15528, 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.

Let us restore the status variable and add a test that lists all InnoDB status variables, to catch similar regressions in the future.