refactor: nettests is now an internal package (#165)

I'm moving everything into internal packages since this ain't a library.

While there, remove typo that was causing a build breakage.

nettests/groups.go

@@ -1,46 +0,0 @@
-package nettests
-
-// NettestGroup base structure
-type NettestGroup struct {
-	Label    string
-	Nettests []Nettest
-}
-
-// NettestGroups that can be run by the user
-var NettestGroups = map[string]NettestGroup{
-	"websites": {
-		Label: "Websites",
-		Nettests: []Nettest{
-			WebConnectivity{},
-		},
-	},
-	"performance": {
-		Label: "Performance",
-		Nettests: []Nettest{
-			Dash{},
-			NDT{},
-		},
-	},
-	"middlebox": {
-		Label: "Middleboxes",
-		Nettests: []Nettest{
-			HTTPInvalidRequestLine{},
-			HTTPHeaderFieldManipulation{},
-		},
-	},
-	"im": {
-		Label: "Instant Messaging",
-		Nettests: []Nettest{
-			FacebookMessenger{},
-			Telegram{},
-			WhatsApp{},
-		},
-	},
-	"circumvention": {
-		Label: "Circumvention Tools",
-		Nettests: []Nettest{
-			Psiphon{},
-			Tor{},
-		},
-	},
-}