Independent components can be exercised by instantiating them independently in a test.
Unit tests for SQL functionality can be exercised via the SQL logic test framework (see pkg/sql/logictest and
Other unit tests that needs a functional SQL or KV layer can use TestServer and TestCluster.
Conversely, avoid using TestServer/TestCluster in unit tests that do not need a full SQL/KV layer. This will make the test run faster.
Integration tests are valuable to exercise a product journey as experienced by an end-user.