Fix random failure in PHP 7 tests "filemtime(): stat failed"

[quicksketch]

Right now we are getting a random failure from PHP 7:

exception filemtime(): stat failed for modules/ddd_installer_test/ddd_installer_test.info system.module:2735

This is likely caused by the PHP stat cache. When installing a module from a remote source, PHP doesn't know that the file exists on disk. It caches it not existing, then when filemtime() is called on it, it fails because the cache contains that file as non-existent.

To fix this, we should clear the file stat cache after installing a remote module in the Installer module.

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PR backdrop/backdrop#1595

[quicksketch]

All tests passed, a good sign. Merged backdrop/backdrop#1595 for 1.5.1.