Skip some tests if xattr support is not available.

This is needed because some builders still lack xattr / fallocate
support despite being updated to Precise.

BUG=chromium:298397
TEST=Remounted the filesystem holding my chroot with nouser_xattr
    (e.g. 'mount -oremount,nouser_xattr /ssd') and unit tests pass
    with the newly added warning.

Change-Id: Id4516477e9b92c6491cc8f1d86078e87d0be942e
Reviewed-on: https://chromium-review.googlesource.com/170870
Reviewed-by: David James <davidjames@chromium.org>
Tested-by: David James <davidjames@chromium.org>
4 files changed