CLN: update imports in tests
Medium-term goal is to separate out tslibs-only parts of these tests to make sure we have sufficient coverage in isolation.
this is fine > This is getting into personal development preferences but as a counter argument I sometimes find these imports very annoying. When debugging the pd namespace is almost always available, yet when copy pasting blocks of code that use this import machinery you then have to go up and see what is imported from top of the module.

+1 on what @WillAyd said here. 
My personal preference is also to use `pd.` in tests when we are testing public things. thanks @jbrockmendel  > I think we as a team have some disagreements over import styles but OK to move forward with this for now and leave that resolution to a larger discussion

Yep, but maybe not do *more* of this PRs before having such discussion then?