selftests: rds: Add helper function send_burst() in test.py

selftests: rds: Add helper function send_burst() in test.py

Hoist the send packet logic in test.py into a helper function,
send_burst().  This is a preparatory refactoring for the rds over ROCE
series that helps modularize the send/recv logic. Breaking up the logic
now will help avoid large function pylint errors later.  No functional
changes are introduced in this patch.

Signed-off-by: Allison Henderson <achender@kernel.org>
Link: https://patch.msgid.link/20260518012443.2629206-5-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>