du_test: Skip three tests if sparse files are not supported

This fixes running the du tests with /tmp as tmpfs (which is what we do in the
CheriBSD CI).

Obtained from: CheriBSD
Reviewed By: ngie
Differential Revision: https://reviews.freebsd.org/D28398

(cherry picked from commit 39a1f858ad735ef816a9b1fb7eeade2c6802eb98)