uclibc: fix testsuite

Unless the tests are built with TEST_INSTALLED_UCLIBC their rpath will
point to the build directory rather than runtime directory, resulting in
non-executable tests for the target.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>