[Fuel backend] Mock LocalCache and REST

[osrf-migration]

Original report (archived issue) by Shane Loretz (Bitbucket: Shane Loretz, GitHub: sloretz).

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The classes LocalCache and REST can be optionally provided to FuelClient. This is meant to allow these classes to be mocked for unit tests. Currently the test directory has gtest, but not gmock, so the classes cannot be mocked. gmock should be added to this repo and unit tests using mocks of these classes should be written to test edge cases that are difficult to test with a real file system or REST server.