refactor(netxlite): improve tests for http and http3 (#487)

* refactor(netxlite): improve tests for http and http3

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

* Update internal/netxlite/http3.go

internal/netxlite/mocks/http3.go

@@ -0,0 +1,19 @@
+package mocks
+
+import "net/http"
+
+// HTTP3RoundTripper allows mocking http3.RoundTripper.
+type HTTP3RoundTripper struct {
+	MockRoundTrip func(req *http.Request) (*http.Response, error)
+	MockClose     func() error
+}
+
+// RoundTrip calls MockRoundTrip.
+func (txp *HTTP3RoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	return txp.MockRoundTrip(req)
+}
+
+// Close calls MockClose.
+func (txp *HTTP3RoundTripper) Close() error {
+	return txp.MockClose()
+}