This change moves the VulkanBenchmark and DrawBenchmark classes to
VulkanWrapper so that they can be used from other unit tests -- namely,
vk-unittests. In doing so, it became clear that using these as base
classes wasn't great for writing googletests, as text fixtures are
classes themselves, and this resulted in messy multiple inheritance. So
I modified the two classes to use callback registration instead of
virtual functions.

Apart from reworking existing tests (e.g. see TriangleBenchmark.cpp), I
also added a new DrawTests.cpp to vk-unittests with a unit test to make
sure we don't crash when leaving out "gl_Position", a bug that sugoi@
fixed in swiftshader-cl/51808. This is a good example of how easy it can
be to write such unit tests now.

List of changes:

* Moved VulkanBenchmark and DrawBenchmark to VulkanWrapper, and renamed
  VulkanTester and DrawTester respectively.
* ClearImageBenchmark refactored to aggregate a VulkanTester. This is an
  example where using a class is fine as we can still use the testers
  via aggregation.
* TriangleBenchmark tests refactored to use DrawTester and register
  callbacks.
* Moved compute tests to a ComputeTests.cpp.
* Moved the other tests to BasicTests.cpp.
* Added DrawTests.cpp with new DrawTests.VertexShaderNoPositionOutput
  test.
* CMake: add VulkanWrapper target for unittests as well as benchmarks.
* CMake: change FOLDER to better organize the tests and benchmarks for
  VS.

Bug: b/176981107
Change-Id: Ib1a0b85b3df787d2e39da08930414f9a14954a73
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52348
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>