oeqa/selftest/tinfoil: Update to use test command

We've had IO load issues on the autobuilder with this test. Avoid
those by using a specilised test command instead.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 106445b1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>