Make ReactorUnitTests use C++17 so we can use std::filesystem.
Since ReactorUnitTests is also built in other build systems, this is
made optional by conditionally compiling based on the __cplusplus macro.

- CMake: ensure that __cplusplus is defined on MSVC
- Also replaced "swiftshader" with "reactor" in the assembly output
filename.
- Added run of unit test to Kokoro.

Bug: b/174358505
Bug: b/174843857
Change-Id: I9c558957f7179e4c295b6d32d78375e18f9e65dd
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50868Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>