Use SHA256 to validate testdata

[reedacartwright]

SHA256 is considered more secure that MD5, so it is a good idea to use it to validate our testdata.tar.gz download.

At the same time, create a make_testdata_sum target to simplify the calculation of the SHA256, e.g.

1. Commit new testing data to the testdata repo.
2. Identify the commit id of the updated testdata repo
3. Edit the TESTDATA_COMMIT in test/CMakeList.txt to reflect the new id.
4. Run make make_testdata_sum to calculate the SHA256 of the data
5. Paste the result into TESTDATA_SHA256 in test/CmakeList.txt
6. Run make testdata to verify that it works.