MAINT - Mock out calls to `conda-lock` for `test_solve_lockfile`

[peytondmurray]

Partially addresses #830.

Description

Currently test_solve_lockfile calls out to conda-lock and also invokes conda info, both of which are quite slow. This PR mocks out calls to conda-lock and conda in conda_store_server/tests/test_actions.py::test_solve_lockfile, and only checks that the invocations are correctly formed, not that conda-lock successfully executes. This reduces the runtime of the test to near zero.

Pull request checklist

• [x] Did you test this change locally?
• [x] Did you update the documentation (if required)?
• [x] Did you add/update relevant tests for this change (if required)?

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