php: add a custom PHP runner for tests

Since the default PHP test runner ignores a good number of environment
variables to potentially tampering the test suite execution, create a
custom php-for-tests.sh script which does nothing more than sourcing the
custom environment that our run-php-tests.sh outputs and running the
actual "php" executable (the one found by configure).

This fixes the loading of the guestfs_php.so module in the test suite,
as the libguestfs.so.0 library can be found by that module.
php: remove the custom "env" loading in tests

Other than being too late for changing environment variables like
LD_LIBRARY_PATH, now it is no more needed.
php: fix crash with OStringList optional arguments

Properly initialize the variables for them to NULL, so there is no crash
because of trying to use rubbish pointers.

Kind of followup of commit 289fd29f0bfbb94566d9780dd8ac3fe15e55541d.