[processing][saga] Remove duplicate symmetrical difference alg

and add test.

Warnings: SAGA results are bad here! Use the native QGIS alg
instead

python/plugins/processing/algs/saga/description/SymmetricalDifference.txt

@@ -1,6 +1,7 @@
 Symmetrical Difference
 shapes_polygons
-QgsProcessingParameterFeatureSource|A|Layer A|-1|None|False
-QgsProcessingParameterFeatureSource|B|Layer B|-1|None|False
+QgsProcessingParameterFeatureSource|A|Layer A|2|None|False
+QgsProcessingParameterFeatureSource|B|Layer B|2|None|False
+QgsProcessingParameterBoolean|SPLIT|Split Parts|True
 QgsProcessingParameterVectorDestination|RESULT|Symmetrical Difference
-QgsProcessingParameterBoolean|SPLIT|Split Parts|True
+

python/plugins/processing/tests/testdata/expected/saga/polygon_symmetrical_difference.mshp

@@ -0,0 +1,30 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<SAGA_METADATA>
+  <HISTORY saga-version="2.3.1">
+    <MODULE library="shapes_polygons" id="16" name="Symmetrical Difference">
+      <OPTION type="boolean" id="SPLIT" name="Split Parts">TRUE</OPTION>
+      <INPUT type="shapes" id="A" name="Layer A">
+        <FILE>/tmp/processing_5303da0b04074aa18f9aa27b64623852/07e557e1ee824d87bb81b1f30a1568be/A.shp</FILE>
+      </INPUT>
+      <INPUT type="shapes" id="B" name="Layer B">
+        <FILE>/tmp/processing_5303da0b04074aa18f9aa27b64623852/e098db0548af4075a60aa2f63b7e6ee6/B.shp</FILE>
+      </INPUT>
+      <OUTPUT type="shapes" id="RESULT" name="Symmetrical Difference">Symmetrical Difference [A]-[B]</OUTPUT>
+    </MODULE>
+  </HISTORY>
+  <SOURCE>
+    <FILE></FILE>
+    <DATABASE>
+      <FIELDS>
+        <FIELD TYPE="STRING">id_a</FIELD>
+        <FIELD TYPE="STRING">id_b</FIELD>
+      </FIELDS>
+    </DATABASE>
+    <PROJECTION>
+      <OGC_WKT>PROJCS["WGS_1984_Web_Mercator_Auxiliary_Sphere",GEOGCS["GCS_WGS_1984",DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137.0,298.257223563]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Mercator_Auxiliary_Sphere"],PARAMETER["False_Easting",0.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",0.0],PARAMETER["Standard_Parallel_1",0.0],PARAMETER["Auxiliary_Sphere_Type",0.0],UNIT["Meter",1.0]]</OGC_WKT>
+      <PROJ4></PROJ4>
+      <EPSG>-1</EPSG>
+    </PROJECTION>
+  </SOURCE>
+  <DESCRIPTION></DESCRIPTION>
+</SAGA_METADATA>

python/plugins/processing/tests/testdata/expected/saga/polygon_symmetrical_difference.prj

@@ -0,0 +1 @@
+PROJCS["WGS 84 / Pseudo-Mercator",GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]],PROJECTION["Mercator_1SP"],PARAMETER["central_meridian",0],PARAMETER["scale_factor",1],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["X",EAST],AXIS["Y",NORTH],EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs"],AUTHORITY["EPSG","3857"]]

python/plugins/processing/tests/testdata/saga_algorithm_tests.yaml

@@ -315,4 +315,19 @@ tests:
         name: expected/saga/polygon_difference.shp
         type: vector
 
+  # Note - SAGA results are wrong here, so really we are just testing that the algorithm runs. Use the native one instead!
+  - algorithm: saga:symmetricaldifference
+    name: Polygon symmetrical difference
+    params:
+      A:
+        name: custom/overlay1_a.geojson
+        type: vector
+      B:
+        name: custom/overlay1_b.geojson
+        type: vector
+      SPLIT: true
+    results:
+      RESULT:
+        name: expected/saga/polygon_symmetrical_difference.shp
+        type: vector