Make missing `__cxa_demangle()` non-fatal

In !658 (merged) I made the unit test for className() actually check the result. That is great and all, but that test is only meaningful if we can actually demangle the type name. It works with all the compilers we have in the CI, still we should not start failing unit tests just because someone is stuck with a compiler that does not have __cxa_demangle().

So:

• (!660 (merged)) skip the result-testing unit test if cmake did not find __cxa_demangle()
• (!666 (merged)) reintroduce the old unit test and run it always so at least the fallback is exercised on compilers without __cxa_demangle()