Revert "Removed unneeded app_label definitions in gis_tests"

This reverts commit e0cc36f615.
The problem is the following:
  * With non-gis backends, gis_tests sub apps are not discovered
    (see runtests.py)
  * Consequently, the app_label is inferred from the gis_tests
    AppConfig
  * Then models with same names in different sub apps conflict
    because of same model_name/app_label pair.

tests/gis_tests/geoadmin/models.py

@@ -10,6 +10,9 @@ class City(models.Model):
 
     objects = models.GeoManager()
 
+    class Meta:
+        app_label = 'geoadmin'
+
     def __str__(self):
         return self.name
 

tests/gis_tests/geoapp/models.py

@@ -24,6 +24,9 @@ class Country(NamedModel):
 class City(NamedModel):
     point = models.PointField()
 
+    class Meta:
+        app_label = 'geoapp'
+
 
 # This is an inherited model from City
 class PennsylvaniaCity(City):
@@ -34,10 +37,16 @@ class PennsylvaniaCity(City):
 
     objects = models.GeoManager()
 
+    class Meta:
+        app_label = 'geoapp'
+
 
 class State(NamedModel):
     poly = models.PolygonField(null=gisfield_may_be_null)  # Allowing NULL geometries here.
 
+    class Meta:
+        app_label = 'geoapp'
+
 
 class Track(NamedModel):
     line = models.LineStringField()

tests/gis_tests/geogapp/models.py

@@ -18,6 +18,9 @@ class NamedModel(models.Model):
 class City(NamedModel):
     point = models.PointField(geography=True)
 
+    class Meta:
+        app_label = 'geogapp'
+
 
 class Zipcode(NamedModel):
     code = models.CharField(max_length=10)
@@ -28,5 +31,8 @@ class County(NamedModel):
     state = models.CharField(max_length=20)
     mpoly = models.MultiPolygonField(geography=True)
 
+    class Meta:
+        app_label = 'geogapp'
+
     def __str__(self):
         return ' County, '.join([self.name, self.state])

tests/gis_tests/layermap/models.py

@@ -36,11 +36,17 @@ class City(NamedModel):
     dt = models.DateField()
     point = models.PointField()
 
+    class Meta:
+        app_label = 'layermap'
+
 
 class Interstate(NamedModel):
     length = models.DecimalField(max_digits=6, decimal_places=2)
     path = models.LineStringField()
 
+    class Meta:
+        app_label = 'layermap'
+
 
 # Same as `City` above, but for testing model inheritance.
 class CityBase(NamedModel):