x/pkgsite: add function to set up frontend server

[julieqiu]

There are several places in the code where we set up a frontend.NewServer:

• cmd/frontend: https://github.com/golang/pkgsite/blob/master/cmd/frontend/main.go#L94-L123
• internal/frontend/server_test.go: https://github.com/golang/pkgsite/blob/master/internal/frontend/server_test.go#L990-L1031
• internal/testing/integration/frontend_test.go: https://github.com/golang/pkgsite/blob/master/internal/testing/integration/frontend_test.go#L155-L167

It would be useful to factor out shared parts of this code to reduce duplication.

[laches1sm]

I'd be happy to try and take on this issue - I'll have a look at the code you provided in the issue and try and refactor it.

[julieqiu]

Please do! As a heads up, I'm about to merge https://go-review.googlesource.com/c/pkgsite/+/240946, which will move around some of the code in internal/testing/integration/frontend_test.go.

[laches1sm]

Awesome, I'll have a wee look at the updated code as soon as you merge :) thank you!