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>

tests/church_6^6.test

@@ -0,0 +1,8 @@
+0 := \f.\x.x
+inc n := \f x.(f (n f x))
+exp n m := (m n)
+print n := (n F X)
+
+N := (inc (inc (inc (inc (inc (inc 0))))))
+
+(print (exp N N))