Improve testing infrastructure with dynamic discovery and validation (#20)

## Summary

This PR enhances the testing infrastructure with dynamic test discovery, automated validation, and improved error handling.

## Changes

### Testing Infrastructure
- Added `TestSamplesValidity` integration test that validates all test files against their expected output.
- Implemented dynamic test discovery using `filepath.Glob` to automatically find all `.test` files.
- Renamed `benchmark_test.go` to `lambda_test.go` for better naming consistency.
- Consolidated helper functions into a single `runSample` function.
- Replaced all error handling with `assert` for consistent and clear test output.
- Required all `.test` files to have corresponding `.expected` files.

### Iterator Improvements
- Added `Swap` method to iterator for better reduction algorithm.
- Improved reduction algorithm with LIFO-based iterator implementation.

### Build System
- Added `make test` target to run tests without benchmarks.
- Updated Makefile help text to include the new test target.

### Test Cases
- Added new test cases with expected outputs: `church_5^5`, `church_6^6`, `fast_list_2^30`, `list_2^30`.
- Added validation files for all test cases.

## Test plan

- Run tests with expected output validation.
- Run benchmarks to ensure performance is maintained.
- Verify make targets work correctly.

Reviewed-on: #20
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

Makefile

@@ -1,7 +1,7 @@
 BINARY_NAME=lambda
 TEST=simple
 
-.PHONY: help build run profile explain graph docs bench clean
+.PHONY: help build run profile explain graph docs test bench clean
 .DEFAULT_GOAL := help
 .SILENT:
 
@@ -13,6 +13,7 @@ help:
 	echo "  explain  - Build and run with explanation mode and profiling"
 	echo "  graph    - Generate and open CPU profile visualization"
 	echo "  docs     - Start local godoc server on port 6060"
+	echo "  test     - Run tests for all samples"
 	echo "  bench    - Run benchmarks for all samples"
 	echo "  clean    - Remove all build artifacts"
 
@@ -38,6 +39,9 @@ docs:
 	echo ">>> View at 'http://localhost:6060/pkg/git.maximhutz.com/max/lambda/'"
 	go run golang.org/x/tools/cmd/godoc@latest -http=:6060
 
+test:
+	go test -v ./cmd/lambda
+
 bench:
 	go test -bench=. -benchtime=10x -cpu=4 ./cmd/lambda