test: prevent running functional tests when not installed

Running functional tests require molecule package to be installed, so
we now test for it and fail fast the testing if needed.

This should simplify development by avoiding confusing errors in
individual tests. Only functional tests are affected so we apply this
requirement only to them.

molecule/test/functional/conftest.py

@@ -40,6 +40,18 @@
 IS_TRAVIS = os.getenv('TRAVIS') and os.getenv('CI')
 
 
+@pytest.fixture(scope="session", autouse=True)
+def require_installed_package():
+    import pkg_resources
+
+    try:
+        pkg_resources.require("molecule")
+    except Exception as e:
+        pytest.fail(
+            "Functional tests require molecule package to be installed: {}".format(e)
+        )
+
+
 def _env_vars_exposed(env_vars, env=os.environ):
     """Check if environment variables are exposed and populated."""
     for env_var in env_vars: