Add `transform` support to `expect_snapshot_file()`

[jennybc]

I just did something janky with expect_snapshot(writeLines(read_utf8(path)), transform = scrub_testpkg) when what I really wanted was expect_snapshot_file(path, transform = scrub_testpkg).

r-lib/usethis@e9a4bec

[hadley]

The fiddly bit is that the defaults for expect_file_snapshot() imply that the files are binary (not text), so it would be a weird if transform was parameterised in the same way as expect_snapshot() (i.e. transforming a character vector). Maybe I can make compare a bit smarter so it can default to NULL and then guess based on the file extension.