docs: end to end testing writing tests. (#14671)
docs: end to end testing writing tests. (#14671)

* docs: end to end testing writing tests.

* Apply suggestions from code review

Added a few commas to improve readability.

Co-Authored-By: gillkyle <94ae90b5a2cda186efc52272b2c47c78482de269@gmail.com>

* Fixed a typo.

* Apply suggestions from code review

Co-Authored-By: Marcy Sutton <63f38fb7a2c9e72439a06d93cefb66621e8fc496@gatsbyjs.com>

* Removed the checkA11y command from the last test.

* Changed the css assertion in the last test.

* Changed the wording of the second test.

* Better intro for automated end-to-end tests.

Co-Authored-By: Marcy Sutton <63f38fb7a2c9e72439a06d93cefb66621e8fc496@gatsbyjs.com>

ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby