Test Helpers: specify spender for SignatureTransfer

[saucepoint]

Similar to https://github.com/Uniswap/permit2/pull/237, this is a more narrowed approach to providing a generalized test helper for SignatureTransfer.

Currently, the test helpers in PermitSignature only generate signatures for a hard-coded address(this).

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Changes:

• I overloaded getPermitTransferSignature to optionally allow users to specify the intended recipient of transferFrom (i.e. a deposit contract)

• added a mock contract to test success/failure paths