Fix CoinBackend testsuite

[jdemeyer]

On 32-bit Linux (Linux arando 3.13.0-95-generic #142-Ubuntu SMP Fri Aug 12 17:05:16 UTC 2016 i686 i686 i686 GNU/Linux) with Sage 7.4.beta4:

sage: from sage.numerical.backends.generic_backend import get_solver
sage: p = get_solver(solver="Coin")
sage: TestSuite(p).run()
Failure in _test_pickling:
Traceback (most recent call last):
  File "/home/jdemeyer/sage-git/local/lib/python2.7/site-packages/sage/misc/sage_unittest.py", line 283, in run
    test_method(tester = tester)
  File "sage/structure/sage_object.pyx", line 620, in sage.structure.sage_object.SageObject._test_pickling (/home/jdemeyer/sage-git/src/build/cythonized/sage/structure/sage_object.c:5233)
    tester.assertEqual(loads(dumps(self)), self)
  File "/home/jdemeyer/sage-git/local/lib/python/unittest/case.py", line 515, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/home/jdemeyer/sage-git/local/lib/python/unittest/case.py", line 508, in _baseAssertEqual
    raise self.failureException(msg)
AssertionError: <type 'sage.numerical.backends.coin_backend.CoinBackend'> != <type 'sage.numerical.backends.coin_backend.CoinBackend'>
------------------------------------------------------------
Failure in _test_solve:
Traceback (most recent call last):
  File "/home/jdemeyer/sage-git/local/lib/python2.7/site-packages/sage/misc/sage_unittest.py", line 283, in run
    test_method(tester = tester)
  File "sage/numerical/backends/generic_backend.pyx", line 697, in sage.numerical.backends.generic_backend.GenericBackend._test_solve (/home/jdemeyer/sage-git/src/build/cythonized/sage/numerical/backends/generic_backend.c:8748)
    with tester.assertRaises(MIPSolverException) as cm:   # unbounded
  File "/home/jdemeyer/sage-git/local/lib/python/unittest/case.py", line 116, in __exit__
    "{0} not raised".format(exc_name))
AssertionError: MIPSolverException not raised
------------------------------------------------------------
The following tests failed: _test_pickling, _test_solve

The _test_pickling failure happens also on 64-bit systems, but the _test_solve failure only on 32-bit.