testenv: avoid dns lookups for *.localhost.pomerium.io (#5372)

* testenv: avoid dns lookups for localhost.pomerium.io

* linter pass

internal/testenv/dns.go

@@ -0,0 +1,64 @@
+package testenv
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"strings"
+	"time"
+)
+
+const localDomainName = "localhost.pomerium.io"
+
+type DialContextFunc = func(ctx context.Context, network string, addr string) (net.Conn, error)
+
+var defaultDialer = &net.Dialer{
+	Timeout:   30 * time.Second,
+	KeepAlive: 30 * time.Second,
+	Resolver: &net.Resolver{
+		PreferGo: true,
+	},
+}
+
+func init() {
+	http.DefaultTransport.(*http.Transport).DialContext = OverrideDialContext(defaultDialer.DialContext)
+}
+
+func OverrideDialContext(defaultDialContext DialContextFunc) DialContextFunc {
+	return func(ctx context.Context, network, addr string) (net.Conn, error) {
+		host, port, err := maybeSplitHostPort(addr)
+		if err != nil {
+			return nil, err
+		}
+		if strings.HasSuffix(host, localDomainName) {
+			switch network {
+			case "tcp", "tcp4", "udp", "udp4":
+				host = "127.0.0.1"
+			case "tcp6", "udp6":
+				host = "::1"
+			}
+		}
+		return defaultDialContext(ctx, network, net.JoinHostPort(host, port))
+	}
+}
+
+func maybeSplitHostPort(s string) (string, string, error) {
+	if strings.Contains(s, ":") {
+		return net.SplitHostPort(s)
+	}
+	return s, "", nil
+}
+
+func GRPCContextDialer(ctx context.Context, target string) (net.Conn, error) {
+	if strings.HasPrefix(target, "unix") {
+		return defaultDialer.DialContext(ctx, "tcp", target)
+	}
+	host, port, err := net.SplitHostPort(target)
+	if err != nil {
+		return nil, err
+	}
+	if strings.HasSuffix(host, localDomainName) {
+		return defaultDialer.DialContext(ctx, "tcp", "127.0.0.1:"+port)
+	}
+	return defaultDialer.DialContext(ctx, "tcp", target)
+}