oeqa/selftest/prservice: Improve debug output for failure

We keep seeing this failure on the autobuilder but the output amounts
to "False is not True". Improve the debug message on the chance it may
make the issue clearer.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d03f4cf1)
Signed-off-by: Steve Sakoman <steve@sakoman.com>