oeqa/runtime/rpm: fail tests if test rpm file cannot be found

oeqa/runtime/rpm: fail tests if test rpm file cannot be found

Discovery of the test file was happening in a class initializer.
That block of code cannot fail (it's not a test), and so it
falls through to completion even if the needed file could not be found.

Then the tests themselves fail later due to class variables not
being set, but all information as to why is already lost at that point.

This converts the discovery to a helper function called from
the tests, so that the function can fail the tests precisely when the
problems occur.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>