selftests: tls: handful of memrnd() and length checks

Add a handful of memory randomizations and precise length checks.
Nothing is really broken here, I did this to increase confidence
when debugging. It does fix a GCC warning, tho. Apparently GCC
recognizes that memory needs to be initialized for send() but
does not recognize that for write().

Signed-off-by: Jakub Kicinski <kuba@kernel.org>