refactor: Migrate external_repo tests to bazel

[furtib]

Why:
We want all tests to be run with bazel test //...

What:

• Migrate external_repository tests to Bazel (This introduces a performance penalty; each external test takes over a minute to run)

Addresses:
#210

[Szelethus]

This introduces a performance penalty; each external test takes over a minute to run

I mean that sounds like a big deal.

[furtib]

Running pytest on the original test takes 55s.
In the Bazel-migrated version, the longest job takes 78s; thanks to parallelization, this is the time for the whole test suite.