[3.15] gh-152168: Don't skip test_bigmem if _testcapi is missing (GH-152171) (#152198)

(cherry picked from commit 6a82832)

Co-authored-by: Shahar Naveh <50263213+ShaharNaveh@users.noreply.github.com>

Author: miss-islington

Lib/test/test_bigmem.py

@@ -10,7 +10,6 @@
 
 from test import support
 from test.support import bigmemtest, _1G, _2G, _4G, import_helper
-_testcapi = import_helper.import_module('_testcapi')
 
 import unittest
 import operator
@@ -1264,6 +1263,7 @@ class ImmortalityTest(unittest.TestCase):
     def test_stickiness(self, size):
         """Check that immortality is "sticky", so that
            once an object is immortal it remains so."""
+        _testcapi = import_helper.import_module('_testcapi')
         if size < _2G:
             # Not enough memory to cause immortality on overflow
             return