Use cli functions

[jayhesselberth]

Part of #2116

[jayhesselberth]

I see this at the end of the build messages, both locally and on GHA. Not sure what is causing this.

Warning messages:
1: In readLines(con, warn = FALSE) :
  invalid input found on input connection '/home/runner/work/pkgdown/pkgdown/vignettes/test/file1a343d741e0a.html'
2: In readLines(con, warn = FALSE) :
  invalid input found on input connection '/home/runner/work/pkgdown/pkgdown/vignettes/test/file1a346eb856ed.html'

[jayhesselberth]

This is ready for review.

[hadley]

Thanks for working on this! It's a big PR so I think it's best to keep up the momentum and get this merged as quickly as possible. We can add follow up issues/PRs for the more complicated stuff.

[jayhesselberth]

These new errors are caused by subtle path differences on the different testing platforms, which fail because the snapshots are slightly different.

I can use expect_message() instead of expect_snapshot() in the failing tests, which works fine, but the actual message bleeds into the interactive testthat report, so I need to do suppressMessages(expect_message(...)), which can't be the best way to do this (though is effective).

[jayhesselberth]

@hadley one more round of review and this should be good to go.