With this change, the flow of TestServer() is simplified: decision logic
whether a test should be run (based on enabled experiments) is moved to
testServer(). Required experiments can be specified when creating
individual test cases in serverTestCases().

This should make the process of adding new tests more transparent
(experiments can be allowed on the test case itself) and remove the need
for returning multiple slices of test cases (for different combinations
of experiments) which are then merged together on a per-test-server
basis. As a trade-off, additional decision logic is added to
testServer() and required experiments have to be specified on each
individual case (as opposed to a block of cases as before).

For golang/go#39663.

Change-Id: Iccdce5ddd7705e1ef8acd8aa4673d4b6a1193b2e
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/243017
Reviewed-by: Julie Qiu <julie@golang.org>