Means to build charm with session scope

[sed-i]

The ops_test fixture has module scope, and it is very convenient to have the model destroyed when I move from one test_*.py to another.
However, I use the same *.charm in all my test_*.py, so fixture such as

@pytest.fixture(scope="module")
async def charm_under_test(ops_test: OpsTest) -> Path:
    """Charm used for integration testing."""
    return = await ops_test.build_charm(".")

would still cause my charm to be built again and again as the test progresses throughout the test_*.py files.

It would be handy to be able to have a charm "persist" without tricks such as

global _path_to_built_charm
if _path_to_built_charm is None:
    _path_to_built_charm = await ops_test.build_charm(".")

Which brings me to a question: I wonder if the design approach is:

1. the higher flexibility of having the entire ops_test in module scope is desirable
2. having the same charm being rebuilt every time is an ok price to pay
3. tests should still be divided into modules rather than one big file with await ops_test.model.reset() at the end of every test function

[PietroPasotti]

+1 on this