CommonJS testing cycle uses stale data

[dibenede]

In the process of generating a PR, I noticed that the GitHub test runners caught issues in commonjs/{import_test.js,strict_test.js} that weren't triggered by my local gulp test_commonjs runs. Closure tests did not seem to be impacted.

E.g. https://github.com/protocolbuffers/protobuf-javascript/actions/runs/3187010981/jobs/5198166279

[dibenede]

Added "clean" commands in #146 to mitigate this a bit.