Test(TileLayer): check zoomOffset option is used (#6011)

following issue #6004 (fixed by PR #6006).
Added 2 dedicated tests.
Unfortunately I could not add a test for detectRetina option that easily, because since the move to Rollup, Browser values became internal variables, and overriding them later on does not have any effect on Leaflet internal references… Not sure how to workaround this for the sake of testing.

spec/suites/layer/tile/TileLayerSpec.js

@@ -377,5 +377,46 @@ describe('TileLayer', function () {
 				expect(['q', 'r', 's'].indexOf(img.src[7]) >= 0).to.eql(true);
 			});
 		});
+
+		it('uses zoomOffset option', function () {
+			// Map view is set at zoom 2 in beforeEach.
+			var layer = L.tileLayer('http://example.com/{z}/{y}/{x}.png', {
+				zoomOffset: 1 // => zoom 2 + zoomOffset 1 => z 3 in URL.
+			}).addTo(map);
+
+			var urls = [
+				'http://example.com/3/1/1.png',
+				'http://example.com/3/1/2.png',
+				'http://example.com/3/2/1.png',
+				'http://example.com/3/2/2.png'
+			];
+
+			var i = 0;
+			eachImg(layer, function (img) {
+				expect(img.src).to.eql(urls[i]);
+				i++;
+			});
+		});
+
+		it('uses negative zoomOffset option', function () {
+			// Map view is set at zoom 2 in beforeEach.
+			var layer = L.tileLayer('http://example.com/{z}/{y}/{x}.png', {
+				zoomOffset: -3 // => zoom 2 + zoomOffset -3 => z -1 in URL.
+			}).addTo(map);
+
+			var urls = [
+				'http://example.com/-1/1/1.png',
+				'http://example.com/-1/1/2.png',
+				'http://example.com/-1/2/1.png',
+				'http://example.com/-1/2/2.png'
+			];
+
+			var i = 0;
+			eachImg(layer, function (img) {
+				expect(img.src).to.eql(urls[i]);
+				i++;
+			});
+		});
+
 	});
 });