ensure containerd io is complete and closed before returning

[coryb]

fixes #2133

The closing of the IO was not synchronized for containerd processes (both Exec and Run). The tests failed ~5% of the time with unexpected output from previous Exec calls because we reuse the buffers but they continued to be written to after the Reset calls. We just need to wait for the IO to complete before returning from Exec/Run. I ran the test 1000 times in a row with no errors:

TESTPKGS="./worker/containerd" TESTFLAGS="-v --count=1000 --parallel=6 --failfast --timeout=120m -run TestContainerdWorkerIntegration/TestContainerdWorkerExec$" ./hack/test integration