cmake: Clean up testing code #1554

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.

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?

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.

utACK 7c987ec89e6cbee5d087683ba29b97012e679484

ACK 7c987ec89e6cbee5d087683ba29b97012e679484

I don't have too much knowledge about CMake, but following the discussion here and reading some CMake forum discussions (e.g. https://discourse.cmake.org/t/is-there-any-reason-to-prefer-include-ctest-or-enable-testing-over-the-other/1905/2), the simplification looks correct.

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.

Verified this by creating a fresh build directory via

$ rm -rf ./build && cmake -B build -DSECP256K1_BUILD_BENCHMARK=OFF -DSECP256K1_BUILD_TESTS=OFF -DSECP256K1_BUILD_EXHAUSTIVE_TESTS=OFF -DSECP256K1_BUILD_CTIME_TESTS=OFF
$ cat ./build/src/CTestTestfile.cmake

and checking that the file only contains comment. If one of these options is ON, the file contains add_test(...) calls.