oe-selftest: Recursively patch test case paths

This ensures that builddir is updated correctly to point to the new
selftest build directory when we're given a list of test suites instead
of a list of test cases.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>