Add CI test stage with coverage reporting and test infrastructure

- Updated .github/workflows/ci.yml to include:
  - Go module caching for faster builds
  - Coverage report generation and upload to Codecov
  - 80% coverage threshold check
- Created tests/ directory with integration test framework
- Added test fixtures and configuration
- Initial integration test passes

Related: FRE-8b42289c (Pop: Add CI test stage to workflow)

tests/README.md

@@ -0,0 +1,28 @@
+# Test Utilities
+
+This directory contains integration test utilities and helpers for the Pop CLI.
+
+## Structure
+
+- `integration_test.go` - Integration test suite
+- `fixtures/` - Test fixtures and test data
+- `helpers/` - Test helper functions
+
+## Running Tests
+
+```bash
+# Run all tests including integration tests
+go test -v ./...
+
+# Run only integration tests
+go test -v ./tests/...
+
+# Run with coverage
+go test -v -coverprofile=coverage.out ./tests/...
+```
+
+## Coverage Requirements
+
+- Minimum 80% coverage required for CI to pass
+- Integration tests should cover end-to-end workflows
+- Unit tests should cover individual components