claim_static_ip does not work for IPADDRESS_TYPE.USER_RESERVED

Tests for claim_static_ip pass a random "IP type." If the random IP type is USER_RESERVED, claim_static_ip will call StaticIPAddress.objects.allocate_new with that type, but without a ‘user’ argument — which triggers an assertion failure.

Example failure (but several tests in the same test case do this):

FAIL: maasserver.models.tests.test_macaddress.TestMACAddressForStaticIPClaiming.test_claim_static_ip_returns_existing_if_claiming_same_type
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_StringException: Traceback (most recent call last):
  File "src/maasserver/models/tests/test_macaddress.py", line 129, in test_claim_static_ip_returns_existing_if_claiming_same_type
    ip = mac.claim_static_ip(alloc_type=iptype)
  File "src/maasserver/models/macaddress.py", line 142, in claim_static_ip
    sip = StaticIPAddress.objects.allocate_new(low, high, alloc_type)
  File "src/maasserver/models/staticipaddress.py", line 71, in allocate_new
    "Must provide user for USER_RESERVED alloc_type")
AssertionError: Must provide user for USER_RESERVED alloc_type