fix(webconnectivity): ignore any status code <= 0 (#579)

This diff changes the algorithm used by webconnectivity's
httpanalysis.go to ignore any status code <= 0 rather
than just ignoring the == 0 case.

Make sure we add test cases for when the control's status
code is negative rather than being zero.

While there, simplify code where boolean checks could be
more compact according to staticcheck.

Closes https://github.com/ooni/probe/issues/1825

internal/engine/experiment/webconnectivity/summary.go

@@ -82,7 +82,7 @@ type Summary struct {
 //
 // Because of that, we must preserve the original behaviour.
 func DetermineBlocking(s Summary) interface{} {
-	if s.Accessible != nil && *s.Accessible == true {
+	if s.Accessible != nil && *s.Accessible {
 		return false
 	}
 	return s.BlockingReason