Stop using sys.args logic in test

[ddoktorski]

Towards #1698

• sys.args logic is unereliable when tests are run on multiple workers using pytest-xdist, each worker executes a separate command which doesn't include intiial --contract_dir value. To fix this we set global _contract_dir variable that is executed before running tests

[ddoktorski]

This change is part of the following stack:

• Add explicit package in load_contract calls #1718
  • Stop using sys.args logic in test #1719 ◀
    • Use new contract in test_declare_v3_tx #1720
      • Fix Windows CI tests #1721

_{Change managed by git-spice.}