selftests/net: Provide test_snprintf() helper

selftests/net: Provide test_snprintf() helper

Instead of pre-allocating a fixed-sized buffer of TEST_MSG_BUFFER_SIZE
and printing into it, call vsnprintf() with str = NULL, which will
return the needed size of the buffer. This hack is documented in
man 3 vsnprintf.

Essentially, in C++ terms, it re-invents std::stringstream, which is
going to be used to print different tracing paths and formatted strings.
Use it straight away in __test_print() - which is thread-safe version of
printing in selftests.

Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240823-tcp-ao-selftests-upd-6-12-v4-2-05623636fe8c@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>