Functional test broken on AppVeyor

This is probably just a race condition in the test -- I think tests run pretty slowly on AppVeyor and so might be more likely to fail when tests rely on timing. A quick glance at the test code makes me think it is susceptible to races in at least two different places.
```
======================================================================
FAIL: test_build_completes_after_allocating_deallocating_and_reallocating_slaves_to_build (test.functional.master.test_deallocation_and_allocation_of_slaves_mid_build.TestDeallocationAndAllocationOfSlavesMidBuild)
----------------------------------------------------------------------
Traceback (most recent call last):
File "C:\projects\clusterrunner\test\functional\master\test_deallocation_and_allocation_of_slaves_mid_build.py", line 30, in test_build_completes_after_allocating_deallocating_and_reallocating_slaves_to_build
self.assert_build_has_successful_status(build_id)
File "C:\projects\clusterrunner\test\framework\functional\base_functional_test_case.py", line 106, in assert_build_has_successful_status
self.assert_build_status_contains_expected_data(build_id, expected_successful_build_params)
File "C:\projects\clusterrunner\test\framework\functional\base_functional_test_case.py", line 92, in assert_build_status_contains_expected_data
'Build status API response should contain the expected status data.')
nose.proxy.AssertionError: Mismatched values: 'result', expected: 'NO_FAILURES', actual: None,'status', expected: 'FINISHED', actual: 'BUILDING' : Build status API response should contain the expected status data.
-------------------- >> begin captured stdout << ---------------------
[2017-05-19 07:12:41.699] 2076 NOTICE MainThread functional_test Hard killing process with pid 2844...
[2017-05-19 07:13:02.944] 2076 NOTICE MainThread functional_test Gracefully killing process with pid 2152...
[2017-05-19 07:13:02.944] 2076 NOTICE MainThread functional_test Gracefully killing process with pid 2908...
--------------------- >> end captured stdout << ----------------------
```