Create mini dataset for tests of functions that use bwtools

[cnluzon]

Right now test_bwplot and test_bwtools use some sort of mix between hard-coding some objects in test_bwtools files and and importing some bigWig / BED files that are deployed with the package in inst/ with bwtools functions for test_bwplot. Plus some mocking to avoid doing this for every function, which would be very slow.

A better solution could be include some RData that can be imported in the tests. Specially for test_bwplot and other things we may add in the future to handle outputs for bwtools functions.

In test_bwtools I'd still keep some degree of hard-coding, because it's then more visible where is the error if some test case crashes, and it becomes easier to add more tests if necessary.