Rewrote dis tests to start a new process to capture stdout (and stderr)

This was chaned because stdout is being written to by rust code, which
currently can't be redirected by reassigning sys.stdout. This caused the
tests to fail because they couldn't see the output of what was being tested.

Lib/test/test_dis.py

@@ -1,29 +1,59 @@
+import subprocess
+import sys
 import unittest
-import dis
-from test.support import captured_stdout
-
 
 # This only tests that it prints something in order
 # to avoid changing this test if the bytecode changes
-class TestDis(unittest.TestCase):
-    def test_dis(self):
-        test_cases = (self.test_dis, "x = 2; print(x)")
 
-        for case in test_cases:
-            with self.subTest(case=case):
-                with captured_stdout() as stdout:
-                    dis.dis(case)
-                self.assertNotEqual("", stdout.getvalue())
+# These tests start a new process instead of redirecting stdout because
+# stdout is being written to by rust code, which currently can't be
+# redirected by reassigning sys.stdout
+
+
+class TestDis(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.setup = """
+import dis
+def tested_func(): pass
+"""
+        cls.command = (sys.executable, "-c")
+
+    def test_dis(self):
+        test_code = f"""
+{self.setup}
+dis.dis(tested_func)
+dis.dis("x = 2; print(x)")
+"""
+
+        result = subprocess.run(
+            self.command + (test_code,), capture_output=True
+        )
+        self.assertNotEqual("", result.stdout.decode())
+        self.assertEqual("", result.stderr.decode())
 
     def test_disassemble(self):
+        test_code = f"""
+{self.setup}
+dis.disassemble(tested_func)
+"""
+        result = subprocess.run(
+            self.command + (test_code,), capture_output=True
+        )
         # In CPython this would raise an AttributeError, not a
         # TypeError because dis is implemented in python in CPython and
         # as such the type mismatch wouldn't be caught immeadiately
-        with self.assertRaises(TypeError):
-            dis.disassemble(self.test_disassemble)
-        with captured_stdout() as stdout:
-            dis.dis(self.test_disassemble.__code__)
-        self.assertNotEqual("", stdout.getvalue())
+        self.assertIn("TypeError", result.stderr.decode())
+
+        test_code = f"""
+{self.setup}
+dis.disassemble(tested_func.__code__)
+"""
+        result = subprocess.run(
+            self.command + (test_code,), capture_output=True
+        )
+        self.assertNotEqual("", result.stdout.decode())
+        self.assertEqual("", result.stderr.decode())
 
 
 if __name__ == "__main__":