Issue #26257: Eliminate buffer_tests.py and fix ByteArrayAsStringTest

ByteArrayAsStringTest.fixtype() was converting test data to bytes, not byte-
array, therefore many of the test cases inherited in this class were not
actually being run on the bytearray type.

The tests in buffer_tests.py were redundant with methods in string_tests
.MixinStrUnicodeUserStringTest and string_tests.CommonTest. These methods are
now moved into string_tests.BaseTest, where they will also get run for bytes
and bytearray.

This change also moves test_additional_split(), test_additional_rsplit(), and
test_strip() from CommonTest to BaseTest, meaning these tests are now run for
bytes and bytearray. I plan to eliminate redundancies with existing tests in
test_bytes.py soon.

Lib/test/test_bytes.py

@@ -1,8 +1,7 @@
 """Unit tests for the bytes and bytearray types.
 
-XXX This is a mess.  Common tests should be moved to buffer_tests.py,
-which itself ought to be unified with string_tests.py (and the latter
-should be modernized).
+XXX This is a mess.  Common tests should be unified with string_tests.py (and
+the latter should be modernized).
 """
 
 import os
@@ -16,7 +15,6 @@ import unittest
 
 import test.support
 import test.string_tests
-import test.buffer_tests
 import test.list_tests
 from test.support import bigaddrspacetest, MAX_Py_ssize_t
 
@@ -1480,8 +1478,7 @@ class AssortedBytesTest(unittest.TestCase):
     # the rest that make sense (the code can be cleaned up to use modern
     # unittest methods at the same time).
 
-class BytearrayPEP3137Test(unittest.TestCase,
-                       test.buffer_tests.MixinBytesBufferCommonTests):
+class BytearrayPEP3137Test(unittest.TestCase):
     def marshal(self, x):
         return bytearray(x)
 
@@ -1511,16 +1508,16 @@ class BytearrayPEP3137Test(unittest.TestCase,
 class FixedStringTest(test.string_tests.BaseTest):
     def fixtype(self, obj):
         if isinstance(obj, str):
-            return obj.encode("utf-8")
+            return self.type2test(obj.encode("utf-8"))
         return super().fixtype(obj)
 
+    contains_bytes = True
+
 class ByteArrayAsStringTest(FixedStringTest, unittest.TestCase):
     type2test = bytearray
-    contains_bytes = True
 
 class BytesAsStringTest(FixedStringTest, unittest.TestCase):
     type2test = bytes
-    contains_bytes = True
 
 
 class SubclassTest: