jiri-test/internal/test/vkube.go: GC old buckets and namespaces
jiri-test/internal/test/vkube.go: GC old buckets and namespaces

The vkube integration test normally cleans up after itself. However,
when the test is interrupted for any reason, the bucket and/or namespace
might be left behind.

With this change, buckets and namespaces that are older than 2 hours are
automatically deleted next time test runs.

Change-Id: Ic8c1831d95f6a2c796e94fef8c2f17e6b93289d2

vanadium/go.devtools,vanadium/go.devtools,vanadium/go.devtools