Fix a couple of issues with the test_structmembersType class in _testcapimodule

 - rename to _test_structmembersType to avoid the class being automatically
   called by test_capi

 - allow space for trailing NUL in inplace_member field of all_structmembers

 - use T_STRING_INPLACE instead of T_INPLACE_STRING as keyword argument
   to _test_structmembersType initializer

 - don't attempt to initialize inplace_member field if T_STRING_INPLACE
   argument wasn't supplied.
diff --git a/Lib/test/test_structmembers.py b/Lib/test/test_structmembers.py
index 279a01e..ff1711c 100644
--- a/Lib/test/test_structmembers.py
+++ b/Lib/test/test_structmembers.py
@@ -1,4 +1,4 @@
-from _testcapi import test_structmembersType, \
+from _testcapi import _test_structmembersType, \
     CHAR_MAX, CHAR_MIN, UCHAR_MAX, \
     SHRT_MAX, SHRT_MIN, USHRT_MAX, \
     INT_MAX, INT_MIN, UINT_MAX, \
@@ -8,7 +8,7 @@
 import unittest
 from test import test_support
 
-ts=test_structmembersType(False, 1, 2, 3, 4, 5, 6, 7, 8,
+ts=_test_structmembersType(False, 1, 2, 3, 4, 5, 6, 7, 8,
                           9.99999, 10.1010101010, "hi")
 
 class ReadWriteTests(unittest.TestCase):