gh-96272: Replace test_source_encoding's test_pep263 with test_import_encoded_module from test_imp (GH-96275)

Editors don't agree that `test_source_encoding.py` was valid koi8-r, making it
hard to edit that file without the editor breaking it in some way (see gh-96272).

Only one test actually relied on the koi8-r encoding and it was a duplicate of a
test from the deprecated `imp` module's `test_imp`, so here we replace
`test_pep263` with `test_import_encoded_module` stolen from `test_imp` and
set `test_source_encoding.py`'s encoding to utf-8 to make editing it easier
going forward.

Lib/test/test_source_encoding.py

@@ -1,4 +1,4 @@
-# -*- coding: koi8-r -*-
+# -*- coding: utf-8 -*-
 
 import unittest
 from test.support import script_helper, captured_stdout, requires_subprocess
@@ -12,15 +12,14 @@ import tempfile
 
 class MiscSourceEncodingTest(unittest.TestCase):
 
-    def test_pep263(self):
-        self.assertEqual(
-            "ðÉÔÏÎ".encode("utf-8"),
-            b'\xd0\x9f\xd0\xb8\xd1\x82\xd0\xbe\xd0\xbd'
-        )
-        self.assertEqual(
-            "\ð".encode("utf-8"),
-            b'\\\xd0\x9f'
-        )
+    def test_import_encoded_module(self):
+        from test.encoded_modules import test_strings
+        # Make sure we're actually testing something
+        self.assertGreaterEqual(len(test_strings), 1)
+        for modname, encoding, teststr in test_strings:
+            mod = importlib.import_module('test.encoded_modules.'
+                                          'module_' + modname)
+            self.assertEqual(teststr, mod.test)
 
     def test_compilestring(self):
         # see #1882