Omit tests folders from coverage
xref #12634 

can you tell me what change this has on the coverage numbers? e.g. going from xxx with tests to yyy w/o?
 Sure. current master 88.68%
https://codecov.io/github/pydata/pandas
With tests excluded 83.63% (subject to change as the build is red ...)
https://codecov.io/github/pydata/pandas?pr=12721
https://codecov.io/github/pydata/pandas?ref=af4e9df23498a6e4ed1980a347aad12a1e4be2a0 (from above in this PR)
 @gliptak thanks!
