[docs] before() is Mocha; beforeEach() is Jest (#14743)
[docs] before() is Mocha; beforeEach() is Jest (#14743)

* before() doesn't exist, but beforeEach() does

https://jestjs.io/docs/en/api

* after() does not exist, but afterEach() does

https://jestjs.io/docs/en/api

* Added comments regarding use of Mocha globals

...not Jest.

* Update docs/src/pages/guides/testing/testing.md

Co-Authored-By: masaok <6d3954e1a2258e6ccdffc174f072be5e2ee8bba8@users.noreply.github.com>

mbrookes/material-ui,allanalexandre/material-ui,rscnt/material-ui,lgollut/material-ui,mbrookes/material-ui,callemall/material-ui,rscnt/material-ui,callemall/material-ui,kybarg/material-ui,oliviertassinari/material-ui,mui-org/material-ui,mbrookes/material-ui,oliviertassinari/material-ui,kybarg/material-ui,callemall/material-ui,kybarg/material-ui,rscnt/material-ui,oliviertassinari/material-ui,mui-org/material-ui,callemall/material-ui,mui-org/material-ui,kybarg/material-ui