twister: fix Ztest C++ test names extraction from ELF

Fix Ztest test function name extraction from ELF symbols
for C++ compiled binaries where symbol names need additional
'demangling' to match with corresponding test names.

The `c++filt` utility (part of binutils) is called for
demangling when it is needed.

Twister test suite extension and adjustment.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>