Test that open files are eventually not leaked
Test that open files are eventually not leaked

By using Eventually, the test now polls for a while to ensure that the
failure condition isn't transient, e.g. if /proc/<pid>/fd was updated
asynchronously to files being closed.

Let's see if this improves the stability of the test.

[#78902702]

shashidharatd/garden-linux,shashidharatd/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,shashidharatd/garden-linux,cloudfoundry-incubator/garden-linux,cloudfoundry-incubator/garden-linux