Address/Thread/UndefinedBehaviourSanitizer-clean unit-tests

We would like to use our unit-tests to find memory errors in our implementation, but for that the tests must not leak memory or contain other memory errors.

This epic is about making our complete test suite asan-, thread- and ubsan-clean.

Note on the cp-demangle.c leaks reported here and there: this is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80578; fixing the UB reported by ubsan should then also remove the leak.

Acceptance criteria:

• all unit-tests report no errors when run on
  • tsan + ubsan
  • asan + ubsan
• the above holds for all tier-1 platforms that support these tools