Provide Examples and Documentation for using MockRos for testing.

[Carter12s]

A huge advantage of roslibrust is that we can write proper node integration tests, run them "as fast as possible" on a machine, and run them in parallel with no "state leaking" to the host. This is especially cool thanks to tokio's tokio::time::pause() feature.

I'd like to expand the examples in roslibrust/examples to specifically show how to "nicely test" a component. Some very basic examples would be good, but I think folks would appreciate some fairly complex examples.

For each example:

• Write a "node" that is Generic on the impl roslibrust::Ros trait
• Have the node do something moderately complicated (ideally with sleeps and delays using tokio::time)
• Use "#[tokio::test]" to set up some tests of that node, either pass in start_paused or invoke tokio::time::pause() on the first line
• Fixture the node with a MockRos instance in the test
• Make calls and assertions in the test
• Measure how long the test takes to run (will be <1 ms probably).