Warn against combining auto test registration and modules with arrays of tests

[martinmoene]

So far found one opportunity to check:

• In lest_MODULE() check for lest_FEATURE_AUTO_REGISTER and warn against auto registration.
• Or define lest_MODULE() as static_assert( false, "...") for auto test registration.
• Or just move lest_MODULE() into non-auto test registration preprocessor else branch.

This doesn't help though to warn against the following incompatible use which you likely encounter earlier:

// for auto test registration
#define CASE( name ) lest_CASE( specification, name );

// non-auto test registration:
static lest::test specification[] =
{
CASE(...) {}
}

See (end of) issue #23 "User-defined functions based on lest macros".