cli: new testhelper and the websteps experiment prototype (#432)

This is the extension of https://github.com/ooni/probe-cli/pull/431, and my final deliverable for GSoC 2021.

The diff introduces:

1) The new `testhelper` which supports testing multiple IP endpoints per domain and introduces HTTP/3 control measurements. The specification of the `testhelper` can be found at https://github.com/ooni/spec/pull/219. The `testhelper` algorithm consists of three main steps:

   * `InitialChecks` verifies that the input URL can be parsed, has an expected scheme, and contains a valid domain name.

   * `Explore` enumerates all the URLs that it discovers by redirection from the original URL, or by detecting h3 support at the target host.

   * `Generate` performs a step-by-step measurement of each discovered URL.

2) A prototype of the corresponding new experiment `websteps` which uses the control measurement of the `testhelper` to know which URLs to measure, and what to expect. The prototype does not yet have:

   * unit and integration tests,

   * an analysis tool to compare the control and the probe measurement.

This PR is my final deliverable as it is the outcome of the trials, considerations and efforts of my GSoC weeks at OONI. 
It fully integrates HTTP/3 (QUIC) support which has been only used in the `urlgetter` experiment until now.

Related issues: https://github.com/ooni/probe/issues/1729 and https://github.com/ooni/probe/issues/1733.

internal/netxlite/resolver.go

@@ -4,6 +4,8 @@ import (
 	"context"
 	"net"
 	"time"
+
+	"golang.org/x/net/idna"
 )
 
 // Resolver performs domain name resolutions.
@@ -80,3 +82,34 @@ func (r *ResolverLogger) Address() string {
 	}
 	return ""
 }
+
+// IDNAResolver is to support resolving Internationalized Domain Names.
+// See RFC3492 for more information.
+type IDNAResolver struct {
+	Resolver
+}
+
+// LookupHost implements Resolver.LookupHost
+func (r *IDNAResolver) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	host, err := idna.ToASCII(hostname)
+	if err != nil {
+		return nil, err
+	}
+	return r.Resolver.LookupHost(ctx, host)
+}
+
+// Network implements Resolver.Network.
+func (r *IDNAResolver) Network() string {
+	if rn, ok := r.Resolver.(resolverNetworker); ok {
+		return rn.Network()
+	}
+	return "idna"
+}
+
+// Address implements Resolver.Address.
+func (r *IDNAResolver) Address() string {
+	if ra, ok := r.Resolver.(resolverAddresser); ok {
+		return ra.Address()
+	}
+	return ""
+}