Automated cmake test discovery

[elv3rs]

Adds a cmake function to query the built fortuno executable and individually register the found tests with ctest.
Supports both post build and pre-test discovery as well as multi configuration generators.

This makes individually registering tests as easy as:

cmake_minimum_required(VERSION 3.25)
project(TestProject LANGUAGES Fortran)

include(FetchContent)
FetchContent_Declare(
    Fortuno
    GIT_REPOSITORY "https://github.com/elv3rs/fortuno"
    GIT_TAG "discovery"
)
FetchContent_MakeAvailable(Fortuno)

enable_testing()

add_executable(testapp testapp.f90)
target_link_libraries(testapp PRIVATE Fortuno::fortuno_serial)

fortuno_discover_tests(testapp)

The fortuno_discover_tests() function is also available after find_package(Fortuno). I.e. if the user is able to link with Fortuno::fortuno..., they can also use fortuno_discover_tests() with no additional includes.