Moved out code from the root CMakeLists into new CMakeLists for each
test and benchmark directory. For Reactor, both the unit test and
benchmark source files are in src/Reactor, so for now, I created folders
for these under tests with a CMakeLists that includes the source files
from src/Reactor. This will be cleaned up eventually when we move
Reactor out of src (b/154013190).

Bug: b/145758253
Change-Id: Ic6bbc394e49d3bdc3105509a19458722fcfe68ee
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43868Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>