pipewire,remoting,tests: Replace asprintf w/ str_printf

We have a string helper which wraps asprintf(). Uses that one because it
clears out the destination string, but also it won't return the number
of bytes unlinke asprintf().

Fixes warnings like:
warning: ignoring return value of ‘asprintf’ declared with attribute
‘warn_unused_result’.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>