tests:Add "--devsim" option

tests:Add "--devsim" option

Add "--devsim" option to vk_layer_validation_tests. This will enable
the Device Simulation layer as the bottom layer on the stack.
It still requires the caller to set the VK_DEVSIM_FILENAME env var in
order to select the json profile that they want to test with.

When "--devsim" option is specified, the device_profile layer will not
be enabled by default when InitFramework() is called. Instead, any
tests that require the device_profile layer can call
EnableDeviceProfileLayer(). This function will cause the device profile
layer to replace the dev sim layer as the bottome layer on the stack.