tapgarden: refactor fundGenesisPsbt to pass in pending batch

Pass the pending batch into fundGenesisPsbt and convert it into a
standalone function rather than a method on ChainPlanter. This change
makes it easier to call fundGenesisPsbt from unit tests.

Author: ffranr

tapgarden/planter.go

@@ -937,8 +937,8 @@ type WalletFundPsbt = func(ctx context.Context,
 // (all outputs need to hold some BTC to not be dust), and with a dummy script.
 // We need to use a dummy script as we can't know the actual script key since
 // that's dependent on the genesis outpoint.
-func (c *ChainPlanter) fundGenesisPsbt(ctx context.Context,
-	batchKey asset.SerializedKey,
+func fundGenesisPsbt(ctx context.Context,
+	pendingBatch *MintingBatch, batchKey asset.SerializedKey,
 	walletFundPsbt WalletFundPsbt) (FundedMintAnchorPsbt, error) {
 
 	var zero FundedMintAnchorPsbt
@@ -947,8 +947,8 @@ func (c *ChainPlanter) fundGenesisPsbt(ctx context.Context,
 	// If universe commitments are enabled, we formulate a pre-commitment
 	// output. This output is spent by the universe commitment transaction.
 	var preCommitmentOut fn.Option[PreCommitmentOutput]
-	if c.pendingBatch != nil && c.pendingBatch.UniverseCommitments {
-		out, err := preCommitmentOutput(c.pendingBatch)
+	if pendingBatch != nil && pendingBatch.UniverseCommitments {
+		out, err := preCommitmentOutput(pendingBatch)
 		if err != nil {
 			return zero, fmt.Errorf("unable to create "+
 				"pre-commitment output: %w", err)
@@ -2044,8 +2044,8 @@ func (c *ChainPlanter) fundBatch(ctx context.Context, params FundParams,
 			return *fundedPkt, nil
 		}
 
-		mintAnchorTx, err := c.fundGenesisPsbt(
-			ctx, batchKey, walletFundPsbt,
+		mintAnchorTx, err := fundGenesisPsbt(
+			ctx, c.pendingBatch, batchKey, walletFundPsbt,
 		)
 		if err != nil {
 			return fmt.Errorf("unable to fund minting PSBT for "+