tests/deqp: add configuration option to control caselist generation

Right now, each time any deqp-like profile ask for the case list, it calls to generate the caselist. As by default it doesn't pass any parameter, it creates all the possible caselists. And there are some test sets (like khr_gles) that calls this method several times. On slow machines this means a significant startup time.

This commit adds a envvar that allows to configure it. The default value is enabled, that keeps the current behaviour. If disabled, if the caselist already exists, it just use it. Note that users using that option are assuming that the caselist is correct, but should regenerate it if they update vk-gl-cts.