TST: Fix answer key index converter for values larger than 'AB'

Change logic so that it uses the 26 ^ n as the base for each letters
index value.
TST: Read benchmark variance from answer key.

Add reference to updated answer key with benchmark variance cells,
and use the new cells as the reference for the benchmark variance
test.

The values changed from the original hardcoded values, due to the
change to close over close benchmarks.