e2e: remove cartesian testing of ipv6 (#6734)

Having looked at our network address parsing and connection code, it
really looks like we're not doing anything on top of what the standard
library is doing (both in terms using `net.ParseIP` and also
`net.Dial`,) and I don't think we need to run the tests 2x the number
of times just to run through different areas of the standard
library. I think most of our users are going to be using IPv4, and
would be down to fully remove this dimension as well, if we find it's
making noise, but for now I think it's fine.

test/e2e/generator/generate.go

@@ -15,7 +15,6 @@ var (
 	// separate testnet for each combination (Cartesian product) of options.
 	testnetCombinations = map[string][]interface{}{
 		"topology":      {"single", "quad", "large"},
-		"ipv6":          {false, true},
 		"p2p":           {NewP2PMode, LegacyP2PMode, HybridP2PMode},
 		"queueType":     {"priority"}, // "fifo", "wdrr"
 		"initialHeight": {0, 1000},
@@ -44,6 +43,7 @@ var (
 	}
 	evidence = uniformChoice{0, 1, 10}
 	txSize   = uniformChoice{1024, 10240} // either 1kb or 10kb
+	ipv6     = uniformChoice{false, true}
 	keyType  = uniformChoice{types.ABCIPubKeyTypeEd25519, types.ABCIPubKeyTypeSecp256k1}
 )
 
@@ -90,7 +90,7 @@ const (
 // generateTestnet generates a single testnet with the given options.
 func generateTestnet(r *rand.Rand, opt map[string]interface{}) (e2e.Manifest, error) {
 	manifest := e2e.Manifest{
-		IPv6:             opt["ipv6"].(bool),
+		IPv6:             ipv6.Choose(r).(bool),
 		InitialHeight:    int64(opt["initialHeight"].(int)),
 		InitialState:     opt["initialState"].(map[string]string),
 		Validators:       &map[string]int64{},