Add test to ensure correct operation of DataGridWidget

tests/integration/fixtures/schema.sql

@@ -4,3 +4,30 @@ CREATE TABLE fixture_item (
 );
 
 INSERT INTO fixture_item (id, name) VALUES (1, 'one'), (2, 'two');
+
+CREATE TABLE grid_parent (
+    id integer PRIMARY KEY,
+    name text NOT NULL
+);
+INSERT INTO grid_parent (id, name) VALUES (1, 'alpha'), (2, 'beta'), (3, 'gamma');
+
+CREATE TABLE grid_child (
+    id integer PRIMARY KEY,
+    parent_id integer,
+    note text
+);
+INSERT INTO grid_child (id, parent_id, note) VALUES (10, 1, 'c1'), (11, 2, 'c2'), (12, 3, 'c3');
+
+CREATE TABLE grid_composite (
+    a integer,
+    b integer,
+    val text,
+    PRIMARY KEY (a, b)
+);
+INSERT INTO grid_composite (a, b, val) VALUES (1, 1, 'x'), (1, 2, 'y'), (2, 1, 'z');
+
+CREATE TABLE grid_nopk (
+    x integer,
+    y text
+);
+INSERT INTO grid_nopk (x, y) VALUES (1, 'a'), (2, 'b');