d/t/test-common: fix finding klibc.so

Florent 'Skia' Jacquetrequested to merge
skia/initramfs-tools:master into master

Since klibc 2.0.13-3, the klibc.*.so shared library is installed in /usr/lib instead of /lib. This patch makes sure the tests are able to find it wherever it lives.

With that patch, a new release would be most welcome.

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