REGTESTS: tcp-rules: read the payload through the deprecated spellings

REGTESTS: tcp-rules: read the payload through the deprecated spellings

The notes listed the historical "payload", "payload_lv" and "req_len"
aliases, without the "req."/"res." prefix, as the only part of this
family still left out. They are read next to the modern spellings and
must report the same values, and since the unprefixed ones work in both
directions, the very same "payload" keyword is also evaluated from a
tcp-response rule on the response buffer.