feat(webconnectivity@v0.5): use TLS info from TH (#933)

This diff modifies webconnectivity@v0.5 to take decisions regarding
TLS blocking by using the response from the TH rather than using
questionable heuristics based on inspecting the TLSHandshake list
alone. This change should improve correctness _when_ we're using
the improved TH, which is currently used for 50% of the probes.

See https://github.com/ooni/probe/issues/2257

While there, modify `control.go` to specify which control is being used.

internal/experiment/webconnectivity/testkeys.go

@@ -66,6 +66,10 @@ type TestKeys struct {
 	// the TH's DNS results and the probe's DNS results.
 	DNSConsistency string `json:"dns_consistency"`
 
+	// HTTPExperimentFailure indicates whether there was a failure in
+	// the final HTTP request that we recorded.
+	HTTPExperimentFailure *string `json:"http_experiment_failure"`
+
 	// BlockingFlags contains blocking flags.
 	BlockingFlags int64 `json:"x_blocking_flags"`