refactor(sessionresolver): adapt to changing network conditions (#238)

* feat(sessionresolver): try many and use what works

* fix(sessionresolver): make sure we can use quic

* fix: the config struct is unnecessary

* fix: make kvstore optional

* feat: write simple integration test

* feat: start adding tests

* feat: continue writing tests

* fix(sessionresolver): add more unit tests

* fix(sessionresolver): finish adding tests

* refactor(sessionresolver): changes after code review

internal/engine/internal/sessionresolver/errwrapper_test.go

@@ -0,0 +1,24 @@
+package sessionresolver
+
+import (
+	"errors"
+	"io"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestErrWrapper(t *testing.T) {
+	ew := &errwrapper{
+		error: io.EOF,
+		URL:   "https://dns.quad9.net/dns-query",
+	}
+	o := ew.Error()
+	expect := "<https://dns.quad9.net/dns-query> EOF"
+	if diff := cmp.Diff(expect, o); diff != "" {
+		t.Fatal(diff)
+	}
+	if !errors.Is(ew, io.EOF) {
+		t.Fatal("not the sub-error we expected")
+	}
+}