Add MAIN_PROJECT check for test option

[globberwops]

Description

When adding doctest to a CMake project using FetchContent, testing has to be explicitly disabled.
This check only enables testing, if doctest is the main project being built.

This change enables the cleaner usage of doctest with FetchContent.

Before

FetchContent_Declare(
  onqtam_doctest
  GIT_REPOSITORY https://github.com/onqtam/doctest.git
  GIT_TAG dev)
set(DOCTEST_WITH_TESTS OFF)
FetchContent_MakeAvailable(onqtam_doctest)

After

FetchContent_Declare(
  onqtam_doctest
  GIT_REPOSITORY https://github.com/onqtam/doctest.git
  GIT_TAG dev)
FetchContent_MakeAvailable(onqtam_doctest)

GitHub Issues

none

[YarikTH]

For reference, here is the same purpose code from {fmt}

# Determine if fmt is built as a subproject (using add_subdirectory)
# or if it is the master project.
set(MASTER_PROJECT OFF)
if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
  set(MASTER_PROJECT ON)
  message(STATUS "CMake version: ${CMAKE_VERSION}")
endif ()
...
...
...
# Options that control generation of various targets.
option(FMT_DOC "Generate the doc target." ${MASTER_PROJECT})
option(FMT_INSTALL "Generate the install target." ${MASTER_PROJECT})
option(FMT_TEST "Generate the test target." ${MASTER_PROJECT})
option(FMT_FUZZ "Generate the fuzz target." OFF)
option(FMT_CUDA_TEST "Generate the cuda-test target." OFF)
option(FMT_OS "Include core requiring OS (Windows/Posix) " ON)

So it might be a good idea at least copy explanatory comment and maybe use CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR check instead of CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR.

[globberwops]

@YarikTH I am having a déjà vu ;) Done 👍🏻

[onqtam]

Thanks for this! merging :)