x/pkgsite: add testcase for stdlib shortcut in frontend.TestServer

[julieqiu]

In frontend.servePackagePageNew, we make a call to s.stdlibPathForShortcut so that the request will redirect if it makes a stdlib shortcut. For example, a request to pkg.go.dev/http will redirect to pkg.go.dev/net/http.

We should add a test case for this in frontend.TestServer.

Related: #39621

[gopherbot]

Change https://golang.org/cl/242137 mentions this issue: internal/frontend: add tests for stdlib shortcuts

[gopherbot]

Change https://golang.org/cl/242138 mentions this issue: internal/frontend: fix redirects for shortcuts with trailing slashes

[gopherbot]

Change https://golang.org/cl/243017 mentions this issue: internal/frontend: refactor serverTestCases()