Fix for bad_shell_exec failing depending on the user's shell

Different shells have different error messages, which trips up
this test case. Switches the test case to a regex covering the
two cases I've seen.

Eg:
```
~/hhvm(branch:master) » echo $SHELL
/usr/bin/zsh

~/hhvm(branch:master) » hphp/hhvm/hhvm hphp/test/run hphp/test/slow/ext_process/bad_shell_exec.php
Running 1 tests in 1 threads
 
FAILED
1 tests failed
(╯°□°）╯︵ ┻━┻
: hphp/test/slow/ext_process/bad_shell_exec.php
002+ sh: 1: /invalid_path/: not found
002- sh: /invalid_path/: No such file or directory
```
I think the space before \/ invalid path needs to be in the optional bracketed part too