feat: clearly indicate which resolver we're using (#885)

See what we documented at https://github.com/ooni/spec/pull/257

Reference issue: https://github.com/ooni/probe/issues/2238

See also the related ooni/spec PR: https://github.com/ooni/spec/pull/257

See also https://github.com/ooni/probe/issues/2237

While there, bump webconnectivity@v0.5 version because this change
has an impact onto the generated data format.

The drop in coverage is unavoidable because we've written some
tests for `measurex` to ensure we deal with DNS resolvers and transport
names correctly depending on the splitting policy we use.

(However, `measurex` is only used for the `tor` experiment and, per
the step-by-step design document, new experiments should use
`measurexlite` instead, so this is hopefully fine(TM).)

While there, fix a broken integration test that does not run in `-short` mode.

internal/netxlite/getaddrinfo_cgo.go

@@ -28,10 +28,13 @@ import (
 // been used to implement the getaddrinfo resolver.
 //
 // This is the CGO_ENABLED=1 implementation of this function, which
-// always returns the string "system", because in this scenario
+// always returns the string [StdlibResolverGetaddrinfo], because in this scenario
 // we are actually calling the getaddrinfo libc function.
+//
+// See https://github.com/ooni/spec/pull/257 for more information on how
+// we evolved our naming of the "stdlib" resolver over time.
 func getaddrinfoResolverNetwork() string {
-	return "system"
+	return StdlibResolverGetaddrinfo
 }
 
 // getaddrinfoLookupANY attempts to perform an ANY lookup using getaddrinfo.