Use helper functions to start neighbor entry tests in their expected state

**Description**

There is currently repeated code within the neighbor entry tests (See `//pkg/tcpip/stack/neighbor_entry_test.go`). This is most evident by the tests that start in the later states, such as Failed. These tests first have to get the entry to transition into Failed state. This works great for being able to track the state transitions all the way through, but adds noise to the tests.

**Do you have a specific solution in mind?**

This issue proposes to use helper functions to transition the neighbor entry to the expected state before testing the intended functionality.