Add integration tests

Tests if repository json files conform to the json schema.
If all resources (images/icons/website URLs) they mention actually
exists.

And can also test writing images and the FAT modification code.

tests/test_urls.py

@@ -0,0 +1,25 @@
+import urllib.request
+
+def _head_request(url):
+    req = urllib.request.Request(
+        url, 
+        data=None, 
+        headers={
+            'User-Agent': 'rpi-imager automated tests'
+        },
+        method="HEAD"
+    )
+    return urllib.request.urlopen(req)    
+
+
+def test_icon_url_exists(iconurl):
+    assert _head_request(iconurl).status == 200
+
+def test_website_url_exists(websiteurl):
+    assert _head_request(websiteurl).status == 200
+
+def test_image_url_exists_and_has_correct_image_download_size(imageitem):
+    response = _head_request(imageitem["url"])
+
+    assert response.status == 200
+    assert str(imageitem["image_download_size"]) == response.headers["Content-Length"]