selftests: tls: add test for peeking past a record of a different type

If we queue 3 records:
 - record 1, type DATA
 - record 2, some other type
 - record 3, type DATA
the current code can look past the 2nd record and merge the 2 data
records.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/4623550f8617c239581030c13402d3262f2bd14f.1708007371.git.sd@queasysnail.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>