Run tests with alternative configuration options in CI? (e.g. --disable-apcu-mmap, protected memory)

[TysonAndre]

Don't test the entire matrix. Maybe just the oldest and newest supported php versions.

Related to #457 - maybe disable serializer=default when apcu is compiled with read only support. Alternately, compute the pointer addresses to use with read only memory when writing to writable memory.

• This may be of some help when debugging and ensuring that apcu's assumptions about when memory is written (e.g. by php-src) are true.
• The benefit of testing protected memory seems negligible for what might be caught, since apcu currently always returns copies of reference counted values such as strings/arrays/references and unserializes data