Loader single EnumPhysDev call through layers

Loader single EnumPhysDev call through layers

The loader trampoline previously would query all devices every time
vkEnumeratePhysicalDevices was called.
To do this, it would make two calls every time:
  - First, it would ignore the passed in user values
  - Second, it would query the total number of available devices.
  - Third, it would query the values for every available device

This resulted in layers reporting 2 vkEnumeratePhysicalDevices call for
every 1 the application made which could get very polluted in output.
It didn't break any functionality, just made things messy.

This change removes that behavior and adds a bunch of test cases to verify
nothing broke in the move.