With this change we no longer can access Vulkan (or Volk) headers
in the ANGLE front-end. The only header that needs to be visible is
the new "DisplayVk_api.h".

This will simplify a bit of Volk integration.

Also reworks how the entry points and headers are exposed to the tests.

Bug: angleproject:3740
Change-Id: Ic1434f304619427ed6ad6370bc090ceb7858df94
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1936707
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tobin Ehlis <tobine@google.com>