tests: move ping tests to src/plugin/ping/test

- Refactor make test code to be co-located with
  the vpp plugin source code

Type: test

Signed-off-by: Dave Wallace <dwallacelf@gmail.com>
Change-Id: I09d55301665449b3f183191537749435f806fb15

diff --git a/test/test_ping.py b/src/plugins/ping/test/test_ping.py
similarity index 100%
rename from test/test_ping.py
rename to src/plugins/ping/test/test_ping.py