shinytest v1.5.1

[schloerke]

• [x] Bump version to 1.5.1
• [x] Update GHA to use rolling R versions
• [x] Check on Ubuntu 20
• [x] Run revdep
• devtools::check_win*()
  • [x] Run system("git clean -xdf --dry-run")
  • [x] Run system("git clean -xdf")
  • [x] Submit devel, release, oldrelease
  • [x] Approve of results
• [x] Update cran-comments.md
• [x] Run system("git clean -xdf")
• [x] devtools::release()
• [x] CRAN accepts
• [x] Merge PR
• [x] Tag commit
• [x] Copy in news to release
• [x] Make dev version on master branch (#411)