heat tests : rename test_stacks

Rename test_stacks.py to test_engine_manager.py
as this better reflects what is being tested

Change-Id: I612351286fbb008b2be3db7519f0a24ab9d2d8e2
Signed-off-by: Steven Hardy <shardy@redhat.com>

diff --git a/heat/tests/test_stacks.py b/heat/tests/test_engine_manager.py
similarity index 100%
rename from heat/tests/test_stacks.py
rename to heat/tests/test_engine_manager.py