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/test_parse_datetime.cpp

@@ -1,4 +1,3 @@
-#define BOOST_TEST_MODULE "parse_datetime_test"
 #include "unit_test.hpp"
 
 #include <toml/parser.hpp>