build(test): remove unnecessary coverage artifacts

[erickzhao]

Follow-up to #3044, this PR removes the Istanbul code coverage tool from our test commands, and de-duplicates the coverage and test commands.

Note
There was a discrepancy in how coverage and test ran fast vs. slow tests.

• coverage ran fast tests with the INTEGRATION_TESTS=0 env var, which specifically disabled the tests/slow/api_spec_slow.ts file.
• test ran fast tests with the TEST_FAST_ONLY env var, which skips any tests that don't end with spec.ts.

I'm not aware of the historical context behind the two flags but it seemed inconsistent to me. I ended up keeping the TEST_FAST_ONLY env var in this PR and removing the other one.