autotest: create get_home_tuple_from_mission method

Useful if you want to start a mission where a saved waypoint file's home location is

Tools/autotest/common.py

@@ -3886,13 +3886,8 @@ class AutoTest(ABC):
             wp.z)
         return wp_int
 
-    def load_mission_from_filepath(self,
-                                   filepath,
-                                   filename,
-                                   target_system=1,
-                                   target_component=1,
-                                   strict=True,
-                                   reset_current_wp=True):
+    def mission_from_filepath(self, filepath, filename, target_system=1, target_component=1):
+        '''returns a list of mission-item-ints from filepath'''
         self.progress("Loading mission (%s)" % filename)
         path = os.path.join(testdir, filepath, filename)
         wploader = mavwp.MAVWPLoader(
@@ -3900,7 +3895,34 @@ class AutoTest(ABC):
             target_component=target_component
         )
         wploader.load(path)
-        wpoints_int = [self.wp_to_mission_item_int(x) for x in wploader.wpoints]
+        return [self.wp_to_mission_item_int(x) for x in wploader.wpoints]
+
+    def get_home_tuple_from_mission(self, filename):
+        '''gets item 0 from the mission file, returns a tuple suitable for
+        passing to customise_SITL_commandline as --home.  Yaw will be
+        0, so the caller may want to fill that in
+        '''
+        items = self.mission_from_filepath(
+            self.current_test_name_directory,
+            filename,
+        )
+        home_item = items[0]
+        return (home_item.x * 1e-7, home_item.y * 1e-7, home_item.z, 0)
+
+    # TODO: rename the following to "upload_mission_from_filepath"
+    def load_mission_from_filepath(self,
+                                   filepath,
+                                   filename,
+                                   target_system=1,
+                                   target_component=1,
+                                   strict=True,
+                                   reset_current_wp=True):
+        wpoints_int = self.mission_from_filepath(
+            filepath,
+            filename,
+            target_system=target_system,
+            target_component=target_component
+        )
         self.check_mission_upload_download(wpoints_int, strict=strict)
         if reset_current_wp:
             # ArduPilot doesn't reset the current waypoint by default