Fix broken tests

[spenczar]

With these two changes, tests pass.

The first change is some minor restructuring of the run_plugin test helper to open files as context managers. This avoids an error related to writing to a closed file at L60.

The second change is to vendor google/protobuf/descriptor.proto from its state as of August 2021 when the golden test output was generated. In the 4 years since then, many more messages and enums were added to descriptor.proto, so the test would fail:

E           dependencies:
E           - name: google/api/http.proto
E             num_dependencies: 0
E             num_messages: 3
E             num_enums: 0
E             num_services: 0
E             num_extensions: 0
E           - name: google/protobuf/descriptor.proto
E             num_dependencies: 0
E         -   num_messages: 33
E         ?                 ^^
E         +   num_messages: 27
E         ?                 ^^
E         -   num_enums: 18
E         ?              ^^
E         +   num_enums: 6
E         ?              ^
E             num_services: 0
E             num_extensions: 0

But this snapshotted old edition fixes that.

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This change is part of a trio. In combination with #18 and #19, github actions should go green again; this is working on my fork's 'actions' branch which merges the three: https://github.com/spenczar/protogen-python/actions/runs/15672639439/