Move all graph tests into integration, because they now rely on the underlying graph driver, which currently cannot be mocked.
Always use the 'vfs' storage driver in integration tests. To test other drivers, we need a dedicated driver validation suite.
Fix a race condition in TestInterruptedRegister
Fix a race condition in the integration tests
Merge pull request #2862 from shykes/integration-tests-vfs

Improve integration tests with vfs driver