tmpnet: Add support for subnets

tests/fixture/tmpnet/network.go

@@ -44,7 +44,7 @@ const (
 	HardHatKeyStr = "56289e99c94b6912bfc12adc093c9b51124f0dc54ac7a766b2bc5ccf558d8027"
 )
 
-// Legacy key used for hardhat testing in subnet-evm
+// HardhatKey is a legacy used for hardhat testing in subnet-evm
 // TODO(marun) Remove when no longer needed.
 var HardhatKey *secp256k1.PrivateKey
 
@@ -157,10 +157,12 @@ func (n *Network) EnsureDefaultConfig(w io.Writer, avalancheGoPath string, plugi
 	n.DefaultFlags.SetDefaults(DefaultFlags())
 
 	// Only configure the plugin dir with a non-empty value to ensure
-	// the use of the default value (`$HOME/.avalanchego/plugins`) when
+	// the use of the default value (`[datadir]/plugins`) when
 	// no plugin dir is configured.
 	if len(pluginDir) > 0 {
-		n.DefaultFlags[config.PluginDirKey] = pluginDir
+		if _, ok := n.DefaultFlags[config.PluginDirKey]; !ok {
+			n.DefaultFlags[config.PluginDirKey] = pluginDir
+		}
 	}
 
 	// Ensure pre-funded keys
@@ -253,17 +255,29 @@ func (n *Network) Create(rootDir string) error {
 	}
 	n.Dir = canonicalDir
 
+	pluginDir, err := n.DefaultFlags.GetStringVal(config.PluginDirKey)
+	if err != nil {
+		return err
+	}
+	if len(pluginDir) > 0 {
+		// Ensure the existence of the plugin directory or nodes won't be able to start.
+		if err := os.MkdirAll(pluginDir, perms.ReadWriteExecute); err != nil {
+			return fmt.Errorf("failed to create plugin dir: %w", err)
+		}
+	}
+
 	if n.Genesis == nil {
 		// Pre-fund known legacy keys to support ad-hoc testing. Usage of a legacy key will
 		// require knowing the key beforehand rather than retrieving it from the set of pre-funded
 		// keys exposed by a network. Since allocation will not be exclusive, a test using a
 		// legacy key is unlikely to be a good candidate for parallel execution.
-		keysToFund := append(
-			n.PreFundedKeys,
+		keysToFund := []*secp256k1.PrivateKey{
 			genesis.VMRQKey,
 			genesis.EWOQKey,
 			HardhatKey,
-		)
+		}
+		keysToFund = append(keysToFund, n.PreFundedKeys...)
+
 		genesis, err := NewTestGenesis(networkID, n.Nodes, keysToFund)
 		if err != nil {
 			return err