Make run-tests script try to find qrexec
Make run-tests script try to find qrexec

Parts of core-admin (specifically some extensions) require qrexec python
module to work, and to be tested. Try to find it in a standard place (as
qubes-builder place it).

QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin