selftests: tls: add test for merging of same-type control messages

Two consecutive control messages of the same type should never be
merged into one large received blob of data.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/018f1633d5471684c65def5fe390de3b15c3d683.1708007371.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>