testcase/checkExecutableLink: tolerate rpath with double slashes

When running with shared parts from slaprunner, we'll have path with
double slashes like:

    /srv/slapgrid/slappart3/srv//runner//shared/libexpat/468993ea3777c1a342102bc1c4b43425/lib/libexpat.so.1

because slaprunner has some paths with double slash.