doc: update frontend docs with new testing commands
doc: update frontend docs with new testing commands

For golang/go#43361

Change-Id: I5303902f29c746638c7a562833be52d3403d972f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/292702
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite