You say ..."I assume I should use regression to solve this" .
I would say "NO" due to the fact that the data is auto-correlated . http://autobox.com/dave/regvsbox.pdf (which I authored) discusses issues/differences/opportunities/pitfalls when dealing with time series that your possible regression solutions may be ignoring.
Edited after receipt of the detailed data 561 stores .... 52 observations per store.
The more I see of the details of 561 (525+36) stores over 52 weeks, the less I am comfortable about analyzing the details (52x561 values). The actual sales for the test group for periods 41-52 could be compared to the expected average sales for that period based upon the actual historical sales of the control group (1-40) and the actual sales for the control group for the period (41-52) using a regression-type approach.
Please post the average sales for the two groups for periods 1-52 as a two column csv file.i.e. 104 values and I will try to help.
As much as I normally ask for all the details , I am at a loss (at the present time) as to how to proceed with a detailed analysis of the 561 samples each with 52 measurements while incorporating temporal effects et al. It smacks of a multi-equational vector ARIMA (561 endogenous series) problem with an exogenous indicator reflecting control vs test and of course potential provisions for unspecified but possibly present and detectable deterministic effects such as pulses/level shifts/local time trends.