[P4Testgen] More P4Tools refactoring work.

[fruffy]

• Move TestUtils.cmake from the testgen folder to the top-level folder. This allows other P4Tools modules to use the same test infrastructure.
• Use the correct folder paths and CMake variables for the CMake test scripts.
• Remove unnecessary code from the P4Tests.cmake files (There was a double file scan, which made cmake calls unnecessarily slow).

Miscellaneous fixes and improvements

• Improve some error messages (for example, state variable matching).
• Add some linter suggestions (capitalization etc).
• Add some new BMv2 constants.
• Require a reference instead of a (potentially null) pointer for table utils.