tests: move gre tests to src/vnet/gre/test

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

Type: test

Signed-off-by: Dave Wallace <dwallacelf@gmail.com>
Change-Id: I4fa716778712ebfb1b136df98775345372d3dfad

diff --git a/test/test_gre.py b/src/vnet/gre/test/test_gre.py
similarity index 100%
rename from test/test_gre.py
rename to src/vnet/gre/test/test_gre.py