perf test - shell - limit to only run executable scripts in tests

perf test's shell runner will just run everything in the tests
directory (as long as it's not another directory or does not begin
with a dot), but sometimes you find files in there that are not shell
scripts - perf.data output for example if you do some testing and then
the next time you run perf test it tries to run these. Check the files
are executable so they are actually intended to be test scripts and
not just some "random junk" files there.