Improve tests from 1625

[montyly]

• Remove the conftest usage
• Use explicit solc version on the tests

#1625 introduced the usage of the conftest, which is great for genericity, however:

• The solution used the global env, instead of the solc select python API
• I think all the tests should use an explicit solc version. Using the "latest" solc if nothing is mentioned is more error-prone
• The logic has a bug, as it does not work well when the tests are re-ordered. I haven't explored why, but it can be shown here: https://github.com/crytic/slither/actions/runs/4383769904/jobs/7674418982