fix: Improve code coverage reporting in codecov (#2715)

* fix: Remove explicit coverpkg github.com/coder/coder/codersdk

This package is already covered by ./...

* fix: Ignore test utils in coverage (clitest, coderdtest, ptytest)

.github/codecov.yml

@@ -34,3 +34,7 @@ ignore:
   - scripts
   - site/.storybook
   - rules.go
+  # Packages used for writing tests.
+  - cli/clitest
+  - coderd/coderdtest
+  - pty/ptytest