e2fsprogs: fix ptest script

e2fsprogs: fix ptest script

* Fix hardcoded path for ptest script which would cause failure on
  mulitilib:
  ls: cannot access '/usr/lib/e2fsprogs/ptest/test/[a-zA-Z]_*': No such file or directory
  ./test_script: line 54: /usr/lib/e2fsprogs/ptest/test/test_post: No such file or directory

* Add missing '$' for shell variable reference

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>