tests: Add infrastructure for y4 tests

tests/map.y4.yml

@@ -0,0 +1,13 @@
+!<tag:y4.managarm.org:preamble>
+import:
+- !std
+---
+result:
+  !std::map
+  list: [1, 2, 3, 4]
+  fn:
+    !std::fn
+    args: [!x ]
+    return: !std::add [!x , 100]
+expected:
+  [101, 102, 103, 104]

tests/reduce.y4.yml

@@ -0,0 +1,14 @@
+!<tag:y4.managarm.org:preamble>
+import:
+- !std
+---
+result:
+  !std::reduce
+  list: [1, 2, 3]
+  init: 100
+  fn:
+    !std::fn
+    args: [!x , !y ]
+    return: !std::add [!x , !y ]
+expected:
+  106

tests/test_y4.py

@@ -0,0 +1,20 @@
+import os
+import pytest
+import subprocess
+import sys
+import yaml
+
+cases = [
+    "map.y4.yml",
+    "reduce.y4.yml",
+]
+
+
+@pytest.mark.parametrize("path", cases)
+def test_y4(path):
+    args = [sys.executable, "-m", "y4", os.path.join("tests", path)]
+    result = subprocess.run(args, stdout=subprocess.PIPE)
+    assert result.returncode == 0
+
+    yml = yaml.load(result.stdout, Loader=yaml.SafeLoader)
+    assert yml["result"] == yml["expected"]