TST: Add condition to linprog test bug 5400

TestLinprogSimplexNoPresolve::test_bug_5400 currently fails on Azure's
Python 3.6 32 bit linux test suite, but, should still pass all other
tests. It was marked with an xfail, that has been modified to only occur
on 32 bit linux systems.

scipy/optimize/tests/test_linprog.py

@@ -2,6 +2,7 @@
 Unit test for Linear Programming
 """
 from __future__ import division, print_function, absolute_import
+import sys
 
 import numpy as np
 from numpy.testing import (assert_, assert_allclose, assert_equal,
@@ -1404,7 +1405,12 @@ class TestLinprogSimplexNoPresolve(LinprogSimplexTests):
     def setup_method(self):
         self.options = {'presolve': False}
 
-    @pytest.mark.xfail(reason='Fails with warning on 32-bit linux')
+    is_32_bit = np.intp(0).itemsize < 8
+    is_linux = sys.platform.startswith('linux')
+
+    @pytest.mark.xfail(
+        condition=is_32_bit and is_linux,
+        reason='Fails with warning on 32-bit linux')
     def test_bug_5400(self):
         super(TestLinprogSimplexNoPresolve, self).test_bug_5400()