Create tool to check skipped tests

[jodh-intel]

Some of our tests employ "skips" to avoid running tests that are known to fail (generally due to open bugs or known limitations). These skips look like this:

• ginkgo:

  Skip("Issue https://github.com/clearcontainers/agent/issues/181")
  

• bats:

  skip "This is not working (https://github.com/clearcontainers/tests/issues/694)"
  

Since we are careful to include an issue number, we could write a tool (check-disabled-tests.{sh,go} ?) that would:

1. Search for all the skips.
2. Warn about any skips that do not contain a URL.
3. Check if the referenced issue is closed.
4. If the issue is closed, warn that the file needs an update to remove the skip.

This tool should be run regularly (ideally as part of every PR CI run, but weekly would probably suffice) to avoid test code that isn't currently being run from going stale, but also to maximise our test coverage.