The free disk space on the Windows RS5 CI nodes appears to be just the
right size that the TestBuildWCOWSandboxSize test can generate 21GB of
layers, and then a 21GB sandbox inside a container, and then runs out of
space while committing the layer.
Helpfully, this failure is distinguishable in the logs from a failure
when the sandbox is too small, so we can do that.
TODO: Revert this if-and-when the Windows-RS5 CI nodes have more free
space.
Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>
695b151a18