Add centre-middle text to set up map frame first

So that we don't plot the map frame four times in a loop.

Author: weiji14

pygmt/tests/baseline/test_text_position.png

@@ -100,13 +100,13 @@ def test_text_nonexistent_filename():
 @pytest.mark.mpl_image_compare
 def test_text_position_four_corners(region):
     """
-    Print text at four corners (top left/right, bottom left/right) of map.
+    Print text at center middle and four corners (top left/right, bottom
+    left/right) of map.
     """
     fig = Figure()
+    fig.text(region=region, projection="x1c", frame="a", position="CM", text="CM")
     for position in ("TL", "TR", "BL", "BR"):
-        fig.text(
-            region=region, projection="x1c", frame="a", position=position, text=position
-        )
+        fig.text(frame="a", position=position, text=position)
     return fig
 
 
@@ -124,19 +124,14 @@ def test_text_xy_with_position_fails(region, projection):
 @pytest.mark.mpl_image_compare
 def test_text_position_offset_with_line(region):
     """
-    Print text at four corners (top left/right, bottom left/right) of map,
-    offset by 0.5 cm, with a line drawn from the original to the shifted point.
+    Print text at centre middle and  four corners (top left/right, bottom
+    left/right) of map, offset by 0.5 cm, with a line drawn from the original
+    to the shifted point.
     """
     fig = Figure()
+    fig.text(region=region, projection="x1c", frame="a", position="CM", text="CM")
     for position in ("TL", "TR", "BL", "BR"):
-        fig.text(
-            region=region,
-            projection="x1c",
-            frame="a",
-            position=position,
-            text=position,
-            offset="j0.5c+v",
-        )
+        fig.text(position=position, text=position, offset="j0.5c+v")
     return fig