test: add erc20 mock with fee on transfer

tests/mocks/MockERC20Fee.vy

@@ -0,0 +1,49 @@
+# pragma version ~=0.4.0
+
+from snekmate.tokens import erc20
+from snekmate.auth import ownable
+
+from ethereum.ercs import IERC20
+
+initializes: ownable
+initializes: erc20[ownable := ownable]
+
+exports: (erc20.approve, erc20.transferFrom)
+
+MAX_FEE: constant(uint256) = 10_000
+FEE_PERCENTAGE: public(immutable(uint256)) 
+
+@deploy
+def __init__(decimals: uint8, fee: uint256):
+    ownable.__init__()
+    erc20.__init__("mock", "mock", decimals, "mock", "mock")
+    assert fee <= MAX_FEE, "fee must be less than or equal to 100%"
+    FEE_PERCENTAGE = fee
+
+
+@internal
+def _transfer(owner: address, to: address, amount: uint256):
+    # Adapted from snekmate to have fee on transfer
+
+    assert owner != empty(address), "erc20: transfer from the zero address"
+    assert to != empty(address), "erc20: transfer to the zero address"
+
+    fee: uint256 = amount * FEE_PERCENTAGE // MAX_FEE
+    amount -= fee
+
+    erc20._burn(owner, fee)
+
+    owner_balance: uint256 = erc20.balanceOf[owner]
+    assert owner_balance >= amount, "erc20: transfer amount exceeds balance"
+    # Deduct full amount from sender
+    erc20.balanceOf[owner] = unsafe_sub(owner_balance, amount)
+    # Credit recipient with full amount first
+    erc20.balanceOf[to] = unsafe_add(erc20.balanceOf[to], amount)
+    log IERC20.Transfer(owner, to, amount)
+
+
+@external
+def transfer(to: address, amount: uint256) -> bool:
+    # Adapted from snekmate to have fee on transfer
+    self._transfer(msg.sender, to, amount)
+    return True