Define BOOST_TEST_MODULE in CMake

This removes one #define from each unit test file and ensures
consistency between file and module names.  This consistency, was not
strictly maintained before.  I hope that any discrepancies were
unintentional and that a 1:1 mapping is actually what is desired.

Since the definition is now done at one single place, it would be easy
to apply transformations like removing the 'test_' prefix or replacing
'_' with '-' if this should be desired.

tests/CMakeLists.txt

@@ -248,6 +248,7 @@ foreach(TEST_NAME ${TEST_NAMES})
     add_executable(${TEST_NAME} ${TEST_NAME}.cpp)
     target_link_libraries(${TEST_NAME} ${Boost_UNIT_TEST_FRAMEWORK_LIBRARY} toml11::toml11)
     target_include_directories(${TEST_NAME} SYSTEM PRIVATE ${Boost_INCLUDE_DIRS})
+    target_compile_definitions(${TEST_NAME} PRIVATE "BOOST_TEST_MODULE=\"${TEST_NAME}\"")
 
     # to compile tests with <filesystem>...
     if(TOML11_REQUIRE_FILESYSTEM_LIBRARY)