assert_enqueued_with with a block ignores all the jobs enqueued before
the block for its assertions by counting the number of jobs and dropping
the n first elements from the Array, but since we're now mutating the
Array in perform_enqueued_jobs without a block, it's broken.

This uses another implementation which is correct when the array is
mutated, by getting a duplicated array of jobs, then removing them from
the original array.

Similarly assert_enqueued_jobs with a block was using counts only, now
keeps track of the specific jobs to count them at the end.