chores: clean up performance issues in tests

[charles-cooper]

Version Information

• vyper Version (output of vyper --version): v0.4.0b2

What's your issue about?

laundry list of performance issues in test suite

• overuse of pytest parametrize (particularly in test_convert.py) results in spending a lot of time in test collection
• eth-tester has inefficient shims, we could modify VyperContract (in conftest.py) to use a different backend
  • we could get rid of get_contract_with_gas_estimation (and either get rid of the gas check entirely or apply it to get_contract so it runs for all tests)

[charles-cooper]

mostly complete in #3846. there are still some things in test collection we can optimize