feat: add failing Test for txmgr edge case

This commit adds a test case for the following scenario:
 1. The first few publications succeed with err == nil.
 2. One of the txs is mined.
 3. Subsequent publications start failing because they are using the
    same nonce as the tx mined in (2).

This edge case is currently overlooked in the txmgr, and will cause the
shouldAbortImmediately method to return true and not wait for the
appropriate confirmations.

go/bss-core/txmgr/txmgr_test.go

@@ -353,7 +353,49 @@ func TestTxMgrConfirmsMinGasPriceAfterBumping(t *testing.T) {
 	}
 
 	ctx := context.Background()
-	receipt, err := h.mgr.Send(ctx, sendTxFunc)
+	receipt, err := h.mgr.Send(ctx, updateGasPrice, sendTx)
+	require.Nil(t, err)
+	require.NotNil(t, receipt)
+	require.Equal(t, h.gasPricer.expGasFeeCap().Uint64(), receipt.GasUsed)
+}
+
+// TestTxMgrDoesntAbortNonceTooLowAfterMiningTx
+func TestTxMgrDoesntAbortNonceTooLowAfterMiningTx(t *testing.T) {
+	t.Parallel()
+
+	h := newTestHarnessWithConfig(configWithNumConfs(2))
+
+	updateGasPrice := func(ctx context.Context) (*types.Transaction, error) {
+		gasTipCap, gasFeeCap := h.gasPricer.sample()
+		return types.NewTx(&types.DynamicFeeTx{
+			GasTipCap: gasTipCap,
+			GasFeeCap: gasFeeCap,
+		}), nil
+	}
+
+	sendTx := func(ctx context.Context, tx *types.Transaction) error {
+		switch {
+
+		// If the txn's gas fee cap is less than the one we expect to mine,
+		// accept the txn to the mempool.
+		case tx.GasFeeCap().Cmp(h.gasPricer.expGasFeeCap()) < 0:
+			return nil
+
+		// Accept and mine the actual txn we expect to confirm.
+		case h.gasPricer.shouldMine(tx.GasFeeCap()):
+			txHash := tx.Hash()
+			h.backend.mine(&txHash, tx.GasFeeCap())
+			return nil
+
+		// For gas prices greater than our expected, return ErrNonceTooLow since
+		// the prior txn confirmed and will invalidate subsequent publications.
+		default:
+			return core.ErrNonceTooLow
+		}
+	}
+
+	ctx := context.Background()
+	receipt, err := h.mgr.Send(ctx, updateGasPrice, sendTx)
 	require.Nil(t, err)
 	require.NotNil(t, receipt)
 	require.Equal(t, h.gasPricer.expGasFeeCap().Uint64(), receipt.GasUsed)