test: template provided through @api-decorated prop (#5243)

* test: template provided through @api-decorated prop

* chore: add comment

packages/@lwc/engine-server/src/__tests__/fixtures/slots-light-evil-template-as-prop/config.json

@@ -0,0 +1,3 @@
+{
+    "entry": "x/outer"
+}

packages/@lwc/engine-server/src/__tests__/fixtures/slots-light-evil-template-as-prop/expected.html

@@ -0,0 +1,15 @@
+<fixture-test>
+  <x-inner>
+    a
+    <!---->
+    slotted content
+    <!---->
+    b
+    <!---->
+    <div>
+      named slotted content
+    </div>
+    <!---->
+    c
+  </x-inner>
+</fixture-test>

packages/@lwc/engine-server/src/__tests__/fixtures/slots-light-evil-template-as-prop/modules/x/inner/inner.js

@@ -0,0 +1,10 @@
+import { LightningElement, api } from 'lwc';
+
+export default class extends LightningElement {
+    static renderMode = 'light';
+    @api tmpl;
+
+    render() {
+        return this.tmpl;
+    }
+}

packages/@lwc/engine-server/src/__tests__/fixtures/slots-light-evil-template-as-prop/modules/x/outer/innerTemplateFn.html

@@ -0,0 +1,7 @@
+<template lwc:render-mode="light">
+    a
+    <slot>fallback for default</slot>
+    b
+    <slot name=foo>fallback for foo</slot>
+    c
+</template>

packages/@lwc/engine-server/src/__tests__/fixtures/slots-light-evil-template-as-prop/modules/x/outer/outer.html

@@ -0,0 +1,6 @@
+<template lwc:render-mode="light">
+    <x-inner tmpl={innerTemplateFn}>
+        slotted content
+        <div slot="foo">named slotted content</div>
+    </x-inner>
+</template>

packages/@lwc/engine-server/src/__tests__/fixtures/slots-light-evil-template-as-prop/modules/x/outer/outer.js

@@ -0,0 +1,14 @@
+import { LightningElement } from 'lwc';
+import innerTemplateFn from './innerTemplateFn.html';
+import tmpl from './outer.html';
+
+export default class extends LightningElement {
+    static renderMode = 'light';
+    innerTemplateFn = innerTemplateFn;
+
+    // This isn't strictly necessary and has no functional value relative to an implicit template. It is present
+    // only to disambiguate the outer component's template from the one we're passing to the child.
+    render() {
+        return tmpl;
+    }
+}