Test for pcntl availability from the command line, not Apache

Summary:
In RHEL6 at the least, pcntl installs from distro package management to the CLI
but not to Apache. Since we don't need it in apache and it's a pain to build
manually, just verify it exists on the CLI.

Test Plan:
Simulated script failures and verified setup output.

Reviewed By: codeblock
Reviewers: codeblock, aran, jungejason, tuomaspelkonen
CC: aran, epriestley, kevinwallace, codeblock
Differential Revision: 380