peekds: create tests for resampling process resample_times

[adriansteffan]

Originally proposed by Martin:

On further inspection, the resampling process shows some odd behavior (e.g., if a chunk of the timecourse behavior is missing, data can be projected backward across the missing chunk - not common in our data but not impossible).

We should write some tests to check the behavior of the approx() function and fix any odd behavior of this kind. A start at these tests is here: https://github.com/langcog/peekds/blob/master/tests/resample_times.R