sim: testsuite: tweak parallel find invocation [PR sim/29596]

Make sure we invoke runtest with the same exp filenames when running in
parallel as it will find when run single threaded.  When `runtest` finds
files itself, it will use paths like "aarch64/allinsn.exp".  When we run
`find .` with the %p option, it produces "./aarch64/allinsn.exp".  Switch
to %P to get "aarch64/allinsn.exp".

Bug: https://sourceware.org/PR29596