[zircon][fvm] Integration test cleanup start.

Move the resize tests into their own translation unit ant test target.
Use zxtest instead of unittest in the new tests.
Create set of utils for common tasks in the resize tests.

Utils treats devices as objects(Using RAII mechanics to close handles
and perform cleanup). This reduces the boilerplate of manually creating
the requests for fvm_* and such free functions, and makes the test more
readable in terms of what actually matters.

New test target is named fvm-integration-test.

TEST=fvm-test,fvm-integration-test

Change-Id: Ie88fb94c61affe188492e1de2991079cffe91d3d