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/list_2^30.test

@@ -0,0 +1,52 @@
+T x y := x
+F x y := y
+if b t e := (b t e)
+
+pair a b := \c.(c a b)
+left p := (p T)
+right p := (p F)
+
+print n := (n 0 1 end)
+
+fix f := (\x.(f (x x)) \x.(f (x x)))
+
+some x := (pair T x)
+none := \.F
+isfull := left
+unwrap := right
+
+nil := none
+push i l := (some (pair i l))
+peek l := (left (unwrap l))
+pop l := (right (unwrap l))
+ 
+inc := (fix \self l.{
+  (if (isfull l)
+    (if (peek l)
+      (push F (self (pop l)))
+      (push T (pop l))
+    )
+    (push T nil)
+  )
+})
+
+print := (fix \self l.{
+  (if (isfull l)
+    ((if (peek l) 1 0) (self (pop l)))
+    END
+  )
+})
+
+one := (push T nil)
+double N := (push F N)
+
+N :=
+  (double (double (double (double (double
+  (double (double (double (double (double
+  (double (double (double (double (double
+  (double (double (double (double (double
+  (double (double (double (double (double
+  (double (double (double (double (double
+  one))))))))))))))))))))))))))))))
+
+(print N)