`pytest-cov` emits hundreds of megabytes of `AstArcAnalyzer`

[imnotjames]

Struggling to create a minimal implementation of this, but it seems that when we're using pytest-cov, mocking out os, and then have an internal pytest error we have hundreds of megabytes of AST data printed out because os.getenv("COVERAGE_TRACK_ARCS") gets accidentally set to a truthy value.

Working on a minimal reproduction..