Add regression tests for geom_quantile()/stat_quantile()

[EK-Lee]

fixes #3135 : add test-geom-quantile.R

[paleolimbot]

Thank you for this PR! Testing the layer_data() of a basic quantile regression plot is exactly what we had in mind. Unfortunately, you will have to update the testing format you've used in order for the checks to pass in ggplot2. In our tests, we don't library() any packages (because our users might not have library()ed them either). If you could:

• Remove the library() calls
• Replace tibble() with data_frame() (to get this to work on your computer, you'll have to run devtools::load_all(.))
• Substitute the pipe and dplyr calls for base R data frame subsetting

We look forward to your changes!

[clauswilke]

Superseded by #3774.