Rather than embedding this, I think it would be more obvious to test the fixture
methods by calling them explicitly - they're just regular methods after all.

e.g. 

    func (*osEnvSuite) TestSetUpSuite(c *gc.C) {
        os.Setenv(....)
        var s testing.OsEnvSuite
        s.SetUpSuite(c)
        assert environment is empty
        s.TearDownSuite(c)
        assert environment has been restored
}