tests: remove unnecessary use of "path_prepend_ ."

tests: remove unnecessary use of "path_prepend_ ."

It is unnecessary, since Makefile.am's TESTS_ENVIRONMENT setting
already adds the top build directory, and besides, is only
marginally useful when running stand-alone, since it presumes
that the script is being run from the top build directory.
* tests/helin-segv: Remove unnecessary use of path_prepend_.
* tests/memcpy-abuse: Likewise.
* tests/trailing-nul: Likewise.
* tests/zdiff: Likewise.
* tests/zgrep-f: Likewise.