Add a new integration test framework

I have had this living in a separate branch for now, but we need this in the main branch sooner rather than later.

One big advantage of this is that integration tests can live in any package, not just hugolib.

hugolib/hugo_sites.go

@@ -117,6 +117,7 @@ func (h *HugoSites) getContentMaps() *pageMaps {
 // Only used in tests.
 type testCounters struct {
 	contentRenderCounter uint64
+	pageRenderCounter    uint64
 }
 
 func (h *testCounters) IncrContentRender() {
@@ -126,6 +127,13 @@ func (h *testCounters) IncrContentRender() {
 	atomic.AddUint64(&h.contentRenderCounter, 1)
 }
 
+func (h *testCounters) IncrPageRender() {
+	if h == nil {
+		return
+	}
+	atomic.AddUint64(&h.pageRenderCounter, 1)
+}
+
 type fatalErrorHandler struct {
 	mu sync.Mutex