netx/{dialer,resolver}: actually add unit tests

They give us confidence that we can successfully log in case of
failure. This is a side effect and I still don't want to make sure
that the output does not change over time. Yet, it's important to
know that the code can handle the format strings we use.

Part of #359

netx/dialer/logging_test.go

@@ -0,0 +1,42 @@
+package dialer_test
+
+import (
+	"context"
+	"crypto/tls"
+	"errors"
+	"io"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-engine/netx/dialer"
+)
+
+func TestUnitLoggingDialerFailure(t *testing.T) {
+	d := dialer.LoggingDialer{
+		Dialer: dialer.EOFDialer{},
+		Logger: log.Log,
+	}
+	conn, err := d.DialContext(context.Background(), "tcp", "www.google.com:443")
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("not the error we expected")
+	}
+	if conn != nil {
+		t.Fatal("expected nil conn here")
+	}
+}
+
+func TestUnitLoggingTLSHandshakerFailure(t *testing.T) {
+	h := dialer.LoggingTLSHandshaker{
+		TLSHandshaker: dialer.EOFTLSHandshaker{},
+		Logger:        log.Log,
+	}
+	tlsconn, _, err := h.Handshake(context.Background(), dialer.EOFConn{}, &tls.Config{
+		ServerName: "www.google.com",
+	})
+	if !errors.Is(err, io.EOF) {
+		t.Fatal("not the error we expected")
+	}
+	if tlsconn != nil {
+		t.Fatal("expected nil tlsconn here")
+	}
+}

netx/resolver/logging_test.go

@@ -0,0 +1,23 @@
+package resolver_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-engine/netx/resolver"
+)
+
+func TestUnitLoggingResolver(t *testing.T) {
+	r := resolver.LoggingResolver{
+		Logger:   log.Log,
+		Resolver: resolver.NewFakeResolverThatFails(),
+	}
+	addrs, err := r.LookupHost(context.Background(), "www.google.com")
+	if err == nil {
+		t.Fatal("expected an error here")
+	}
+	if addrs != nil {
+		t.Fatal("expected nil addr here")
+	}
+}