Review test-stat* and test-theme

[zeehio]

• I added all the ggplot tests from 2.2.1
• I reviewed the tests, removing those testing ggplot internals
• I simplified the tests, testing only for equality between ggplot objects generated by ggpipe and ggplot (the plot building phase is not touched by ggpipe)

The last step (simplify the tests) was not done for the test-stat* and test-theme* tests, so I leave that for the future, before a CRAN release.