Reduce devices created in device_group tests

Reduce devices created in device_group tests

The device_group tests could sometimes hit timeouts on
slower CPUs. This change reduces the number of objects
created in these tests to match the reduced number used
in the device and instance tests.

Affects:
dEQP-VK.api.object_management.
	multithreaded_per_thread_resources.device_group
dEQP-VK.api.object_management.
	multithreaded_shared_resources.device_group

Components: Vulkan

VK-GL-CTS issue: 1512
Change-Id: Icb68643ff530a8b1a1a7daa9fc93d8db8bc5faf9