Use PHP_BINARY Constant for run-tests.sh

Hi @generalredneck:

It doesn't break anything on a Mac, or the Linux testbots. This seems fine to me. But there is a backport policy so we don't miss useful enhancements on newer versions. These sections of the policy are relevant to this issue:

Where an issue filed against 8.x also applies to 7.x, a separate issue should be opened against 7.x, related to 8.x via the 'Parent issue' field (this can be done using the "Add child issue" link on the 8.x issue).
... By default, 7.x issues should be marked postponed until the 8.x issue is fixed to avoid duplicate work.

There already is an issue open in D8, #2748883: Use the PHP_BINARY constant in addition to --php in run-tests.sh. I am marking this postponed on that one.

D8 is set to use Symphony's PhpExecutableFinder, no chance of duplicate work, so we're unblocked.

#2748883: Use the PHP_BINARY constant in addition to --php in run-tests.sh is still open for D8 (likely not getting much attention as I'm not sure the run-tests.sh script is used much now in newer branches), but as noted by @MustangGB the suggested approach there is to use Symfony's

https://github.com/symfony/process/blob/3.4/PhpExecutableFinder.php

#2 looks okay to me, but if we're primarily concerned with fixing run-tests.sh for Windows (as it works okay on *nix hosts which set the shell environment up in a reasonably consistent way), couldn't we put the new logic in later as a fallback? - e.g. something like:

  // Determine location of php command automatically, unless a command line argument is supplied.
  if (!empty($args['php'])) {                                                      
    $php = $args['php'];                                                           
  }                                                                                
  elseif ($php_env = getenv('_')) {                                                
    // '_' is an environment variable set by the shell. It contains the command that was executed.
    $php = $php_env;                                                               
  }
  elseif (defined('PHP_BINARY') && $php_env = PHP_BINARY) {
    $php = $php_env;
  }                                                                         
  elseif ($sudo = getenv('SUDO_COMMAND')) {

Perhaps that's a very minor point, but in the interests of "if it ain't broke, don't fix it" ...

This is hardly any different to #2 but will use the shell's _ env variable first if that's available.

RTBM, +1 from me for merge

Thanks everyone!