More tests in Sentry CLI

[szokeasaurusrex]

We should add more tests to the Sentry CLI, to ensure we are thoroughly testing common usage scenarios and edge cases. We especially need to test sourcemaps inject more thoroughly, but all areas of the CLI could use more testing.

We might also consider testing the CLI in different ways. Currently, most of the tests are integration tests that check the exact output of a command. Perhaps, we could add more unit tests to test narrower portions of the code, and to make it easier to have parameterized test cases where we run similar tests with slightly different parameters.

It will likely make sense to split this issue into several sub-issues.