Add tests for slots in custom elements

test/validator/samples/general-siblings-combinator-in-custom-element-selects-slot-fallback/_config.js

@@ -0,0 +1,3 @@
+export default {
+	customElement: true
+};

test/validator/samples/general-siblings-combinator-in-custom-element-selects-slot-fallback/input.svelte

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+<svelte:options tag="my-element" />
+
+<h1>Heading 1</h1>
+<span>Span 1</span>
+<span>Span 2</span>
+<slot>
+	<p>Paragraph 2</p>
+</slot>
+
+<style>
+	/* This will not get picked up */
+	h1 ~ p {
+		color: red;
+	}
+
+	/* This will be picked up */
+	h1 ~ slot > p {
+		color: red;
+	}
+</style>

test/validator/samples/general-siblings-combinator-in-custom-element-selects-slot-fallback/warnings.json

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+[
+  {
+    "code": "css-unused-selector",
+    "message": "Unused CSS selector \"h1 ~ p\"",
+    "start": {
+      "column": 1,
+      "line": 12
+    },
+    "end": {
+      "column": 7,
+      "line": 12
+    }
+  }
+]

test/validator/samples/siblings-combinator-in-custom-element-selects-slot-fallback/_config.js

@@ -0,0 +1,3 @@
+export default {
+	customElement: true
+};

test/validator/samples/siblings-combinator-in-custom-element-selects-slot-fallback/input.svelte

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+<svelte:options tag="custom-element" />
+
+<h1>test</h1>
+<slot>
+  <span>Hello</span>
+</slot>
+
+<style>
+	/* This will not be picked up */
+  h1 + span {
+    color: red;
+  }
+
+	/* This will be picked up */
+	h1 + slot > span {
+		color: red;
+	}
+</style>

test/validator/samples/siblings-combinator-in-custom-element-selects-slot-fallback/warnings.json

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+[
+  {
+    "code": "css-unused-selector",
+    "end": {
+      "column": 11,
+      "line": 10
+    },
+    "message": "Unused CSS selector \"h1 + span\"",
+    "start": {
+      "column": 2,
+      "line": 10
+    }
+  }
+]