Add initial test scaffolding

src/lib/test-utils/TestRenderer.svelte

@@ -0,0 +1,43 @@
+<script lang="ts" context="module">
+  type HandlerType = (event?: CustomEvent) => any;
+  interface ComponentProps {
+    onChange?: HandlerType;
+  }
+  type SingleComponent = [SvelteComponent, ComponentProps, TestRendererProps];
+  export type TestRendererProps =
+    | undefined
+    | string
+    | SingleComponent
+    | SingleComponent[];
+</script>
+
+<script lang="ts">
+  import type { SvelteComponent } from "svelte";
+
+  function isSingleComponent(
+    props: SingleComponent | SingleComponent[]
+  ): props is SingleComponent {
+    return Array.isArray(props) && !Array.isArray(props[0]);
+  }
+  export let allProps: TestRendererProps;
+  let onChange: HandlerType = () => {};
+  if (allProps && typeof allProps !== "string" && isSingleComponent(allProps)) {
+    if (allProps[1].onChange) {
+      onChange = allProps[1].onChange;
+    }
+  }
+</script>
+
+{#if typeof allProps === "string"}
+  {allProps}
+{:else if Array.isArray(allProps)}
+  {#if Array.isArray(allProps[0])}
+    {#each allProps as childProps}
+      <svelte:self allProps={childProps} />
+    {/each}
+  {:else}
+    <svelte:component this={allProps[0]} {...allProps[1]} on:change={onChange}>
+      <svelte:self allProps={allProps[2]} />
+    </svelte:component>
+  {/if}
+{/if}