Allow samples tests to run concurrently

[igorbernstein2]

It seems like the current samples tests hardcode their resource ids. Which makes them clobber each other when the tests are run concurrently (ie. multiple PRs are open).

We should mitigate this by either:

1. using the same strategy as the general integration tests and using timestamped resource names
2. update kokoro to take a lock for the samples tests

I would prefer 1

[billyjacobson]

The table and instance admin tests create all new resources and the reads/filters/writes tests use the same instance but create tables with dynamically generated names. There isn't anything too intense going on in any of these tests though, so it shouldn't be having issues. Let me know if there are some specific instances you've seen.

I like having tests use a pre-existing instance since it is faster and doesn't run into issues with instances not being deleted and having dozens of unused nodes in random projects.