Unit tests fail when build/test box has an eth1 interface

It seems that some unit tests expect that the build box does not have an eth1 interface. My build box happened to have an eth1 interface, and running into this was slightly confusing. Unit tests that expect an interface to NOT exist should probably use non-default interface names.

Relevant output:

[ RUN      ] CfgSubnets4Test.iface
cfg_subnets4_unittest.cc:1777: Failure
Expected: parser.parse(elems) throws an exception of type DhcpConfigError.
  Actual: it throws nothing.
[  FAILED  ] CfgSubnets4Test.iface (0 ms)
[ RUN      ] SharedNetwork4ParserTest.iface
shared_network_parser_unittest.cc:472: Failure
Expected: parser.parse(config_element) throws an exception of type DhcpConfigError.
  Actual: it throws nothing.
[  FAILED  ] SharedNetwork4ParserTest.iface (0 ms)
[ RUN      ] SharedNetwork4ParserTest.iface
shared_network_parser_unittest.cc:472: Failure
Expected: parser.parse(config_element) throws an exception of type DhcpConfigError.
  Actual: it throws nothing.
[  FAILED  ] SharedNetwork4ParserTest.iface (0 ms)
[ RUN      ] SharedNetwork6ParserTest.iface
shared_network_parser_unittest.cc:1007: Failure
Expected: parser.parse(config_element) throws an exception of type DhcpConfigError.
  Actual: it throws nothing.
[  FAILED  ] SharedNetwork6ParserTest.iface (0 ms)
[  FAILED  ] 4 tests, listed below:
[  FAILED  ] CfgSubnets4Test.iface
[  FAILED  ] CfgSubnets6Test.iface
[  FAILED  ] SharedNetwork4ParserTest.iface
[  FAILED  ] SharedNetwork6ParserTest.iface