Lifecycle's test_core_snap environment gets corrupted by something earlier in the suite

If I run test_core_snap on its own, it works fine. However, when I run it with the rest of the suite, something appears to be corrupting its path:

======================================================================
ERROR: test_core_setup (snapcraft.tests.test_lifecycle.CoreSetupTestCase)
snapcraft.tests.test_lifecycle.CoreSetupTestCase.test_core_setup
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testtools.testresult.real._StringException: Empty attachments:
  pythonlogging:''

Traceback (most recent call last):
  File "/usr/lib/python3.5/unittest/mock.py", line 1157, in patched
    return func(*args, **keywargs)
  File "/home/kyrofa/src/snapcraft/snapcraft/tests/test_lifecycle.py", line 664, in test_core_setup
    core_snap = self._create_core_snap()
  File "/home/kyrofa/src/snapcraft/snapcraft/tests/test_lifecycle.py", line 732, in _create_core_snap
    return lifecycle.snap(self.project_options, directory=core_path)
  File "/home/kyrofa/src/snapcraft/snapcraft/internal/lifecycle.py", line 385, in snap
    stdout=PIPE, stderr=STDOUT) as proc:
  File "/usr/lib/python3.5/subprocess.py", line 947, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.5/subprocess.py", line 1551, in _execute_child
    raise child_exception_type(errno_num, err_msg)
FileNotFoundError: [Errno 2] No such file or directory: 'mksquashfs'