SetupTestOSContextEx calls 'ns.Init' (which, outside tests, is protected
by a sync.Once), and it's called again by the returned OSContext.Cleanup
method. That overwrites the ns package's namespace and netlink handles
(initNs and initNl) without closing them.
Because SetupTestOSContextEx changes that shared state, it should not
be used in parallel tests. So, rather than trying to close the handles
in ns.Init if already open - un-export Init so it's always called via
its sync.Once, and add a reset function for tests to use. Have
SetupTestOSContextEx claim a mutex to avoid crashy surprises or
hard to catch issues where the ns package isn't using the expected
namespace if it is used in parallel tests.
Signed-off-by: Rob Murray <rob.murray@docker.com>
