Snappi sample test (#3653)

* Sample traffic test using snappi
* Snappi helpers for traffic generation

What is the motivation for this PR?
The motivation for this PR is to migrate the tests that are written with ixnetwork open traffic generator to snappi

How did you do it?
Replaced the code where abstract is used to snappi

How did you verify/test it?
verified the tests in sonic-mgmt using pytest with DUT and IXIA connections
snappi version used for the development of tests snappi[ixnetwork]==0.4.14

tests/common/snappi/__init__.py

@@ -0,0 +1 @@
+# Place for snappi fixtures.