refactor(netxlite): add more functions to resolver (#455)

We would like to refactor the code so that a DoH resolver owns the
connections of its underlying HTTP client.

To do that, we need first to incorporate CloseIdleConnections
into the Resolver model. Then, we need to add the same function
to all netxlite types that wrap a Resolver type.

At the same time, we want the rest of the code for now to continue
with the simpler definition of a Resolver, now called ResolverLegacy.

We will eventually propagate this change to the rest of the tree
and simplify the way in which we manage Resolvers.

To make this possible, we introduce a new factory function that
adapts a ResolverLegacy to become a Resolver.

See https://github.com/ooni/probe/issues/1591.

internal/netxlite/mocks/resolver.go

@@ -4,9 +4,10 @@ import "context"
 
 // Resolver is a mockable Resolver.
 type Resolver struct {
-	MockLookupHost func(ctx context.Context, domain string) ([]string, error)
-	MockNetwork    func() string
-	MockAddress    func() string
+	MockLookupHost           func(ctx context.Context, domain string) ([]string, error)
+	MockNetwork              func() string
+	MockAddress              func() string
+	MockCloseIdleConnections func()
 }
 
 // LookupHost calls MockLookupHost.
@@ -23,3 +24,8 @@ func (r *Resolver) Address() string {
 func (r *Resolver) Network() string {
 	return r.MockNetwork()
 }
+
+// CloseIdleConnections calls MockCloseIdleConnections.
+func (r *Resolver) CloseIdleConnections() {
+	r.MockCloseIdleConnections()
+}

internal/netxlite/mocks/resolver_test.go

@@ -44,3 +44,16 @@ func TestResolverAddress(t *testing.T) {
 		t.Fatal("unexpected address", v)
 	}
 }
+
+func TestResolverCloseIdleConnections(t *testing.T) {
+	var called bool
+	r := &Resolver{
+		MockCloseIdleConnections: func() {
+			called = true
+		},
+	}
+	r.CloseIdleConnections()
+	if !called {
+		t.Fatal("not called")
+	}
+}