gstrfuncs: Fix regression in C++ types accepted by g_str_equal()

Further to commit bcd364af

, fix the types accepted by the
`g_str_equal()` macro for C++ too. C++ is more restrictive about
const-correctness.

Add unit tests.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2820