Shebang now works on both Linux and Mac OS X.

* Updated the shebangs of both fix and the test script to use 'env'.
  This requires the binary '/usr/bin/env' to exist on the system, but
  allows the perl interpreter to exist anywhere in $PATH.

* Improved the $PATH related greybox tests. (These tests failed
  miserably with the new env shebang, since they cleared the $PATH
  variable in order before invoking fix, whereupon env had nowhere to
  look for perl. Causing fix to fail to execute.) Rewrote the tests to
  work without clearing $PATH.