Partial tests for FrontLine.

We need to mock the backend to usefully test the contextmenu handler.
I'd like to finish all the low hanging fruit before going for that.

client/src/components/frontline/FrontLine.test.tsx

@@ -0,0 +1,32 @@
+import { renderWithProviders } from "../../testutils";
+import FrontLine from "./FrontLine";
+import { PolylineProps } from "react-leaflet";
+
+const mockPolyline = jest.fn();
+jest.mock("react-leaflet", () => ({
+  Polyline: (props: PolylineProps) => {
+    mockPolyline(props);
+  },
+}));
+
+describe("FrontLine", () => {
+  it("is drawn in the correct location", () => {
+    const extents = [
+      { lat: 0, lng: 0 },
+      { lat: 1, lng: 0 },
+    ];
+    renderWithProviders(
+      <FrontLine
+        front={{
+          id: "",
+          extents: extents,
+        }}
+      />
+    );
+    expect(mockPolyline).toHaveBeenCalledWith(
+      expect.objectContaining({
+        positions: extents,
+      })
+    );
+  });
+});