Fix/ladspa test
Ladspa quiddity is saving parameter values as string serialized from various types, including floats. However, switcher is using std::to_string, which is locale dependant. This MR forces ladspa plugin to switch to the "C" locale before serialization, and then restore the previous one.
The check_ladspa syntax test has been simplified, since it was made obscure after several automatic refactorings.