fix startup with the default ffi.enable=preload

[wadakatu]

On a stock PHP install ffi.enable is preload, and since #287 php-vips refuses to start there even though FFI works:

$ php -r 'var_export(ini_get("ffi.enable"));'
'preload'
$ php -r 'require "vendor/autoload.php"; echo Jcupitt\Vips\Config::version(), "\n";'
Fatal error: Uncaught Jcupitt\Vips\Exception: ffi.enable set to 'preload', not 'true'
$ php -r 'FFI::cdef("int puts(const char *);", "libc.so.6")->puts("FFI works");'
FFI works

With ffi.enable=preload, PHP allows an FFI call if the SAPI is cli, or the caller was compiled during opcache.preload, or the preload script is running (zend_ffi_validate_api_restriction() in ext/ffi/ffi.c). Only the SAPI is visible from PHP, so no check on the ini value can be right: the same value works under php -r and is refused under php -S.

• Drop the check and let PHP decide. extension_loaded("ffi") stays.
• libraryLoad() was catching the \FFI\Exception and dropping it unless a logger was set, so init() blamed a missing libvips whatever the cause (ffi.enable check incorrect, default value passes it, even if not enabled #286). Keep the last one, append its message to the startup exception and pass it as $previous.

ffi.enable is INI_SYSTEM, so StartupTest runs a second PHP process with -d ffi.enable=preload and -d ffi.enable=0. Both tests fail on master. phpunit.xml now uses the default Test.php suffix so the helper script is not picked up as a test.