Remove input file from BDD test context when reading inline OSM data

If an input file was read first and in another step inline OSM data, the
context still knew about the file and re-read it.

tests/bdd/steps/steps_osm_data.py

@@ -31,6 +31,7 @@ def osm_define_node_grid(context, step, origin_x, origin_y):
 
 @given("the (?P<formatted>python-formatted )?OSM data")
 def osm_define_data(context, formatted):
+    context.import_file = None
     data = context.text
     if formatted:
         data = eval('f"""' + data + '"""')