* Rename variable from thing to contractUnderTest

* Compute function signatures in ERC165 interfaces

The ERC165 tests currently precompute some known interface ids.
This commit extracts the interfaces into a separate object and
precomputes the individual function signatures.

This will be useful to identify contracts that support an interface
but do not implement all of the corresponding functions.

* Add tests for ERC165 interface implementations

The ERC165 tests confirm that contracts claim to support
particular interfaces ( using the supportsInterface method )

This commit extends those tests to confirm that the corresponding
functions are included in the contract ABI.

It also rewords the existing test names in order to group the
implementation tests with the corresponding interface tests.

* Remove obsolete ERC721Exists interface constant