cmake: Clean up testing code

[hebasto]

1. Delete CTest module.

The CTest module handles CDash integration, which we do not use. It is not required for testing functionality.

2. Clean up cases when to invoke enable_testing()

The enable_testing() command invocation is required for add_test() commands, which are used only for {noverify_}tests, exhaustive_tests and examples.

[real-or-random]

The enable_testing() command invocation is required for add_test() commands, which are used only for {noverify_}tests, exhaustive_tests and examples.

I don't think it's required. It's just that add_test() has no effect without enable_testing().

enable_testing() seems cheap. Couldn't we just run it always? Or do you think this has drawbacks?

[hebasto]

The enable_testing() command invocation is required for add_test() commands, which are used only for {noverify_}tests, exhaustive_tests and examples.

I don't think it's required. It's just that add_test() has no effect without enable_testing().

enable_testing() seems cheap.

I agree.

Couldn't we just run it always?

Sure. The code has been adjusted.

Or do you think this has drawbacks?

I did some research and I haven't found any drawbacks. When enable_testing() is invoked and no tests being added, the created CTestTestfile.cmake files in the binary tree are noop/empty.

It is recommended:

to call enable_testing() somewhere in the top level CMakeLists.txt file. This would typically be done early, soon after the first project() call.