Add tests for having slots between former and latter element

test/css/samples/general-siblings-combinator-slots-between/expected.css

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+h1.svelte-xyz~p.svelte-xyz{color:red}

test/css/samples/general-siblings-combinator-slots-between/input.svelte

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+<h1>Heading 1</h1>
+<slot>
+  <span>Span 1</span>
+</slot>
+<slot>
+  <span>Span 2</span>
+</slot>
+<p>Paragraph 2</p>
+
+<style>
+  h1 ~ p {
+    color: red;
+  }
+</style>

test/css/samples/siblings-combinator-slots-between/expected.css

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+h1.svelte-xyz+span.svelte-xyz{color:red}

test/css/samples/siblings-combinator-slots-between/input.svelte

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+<h1>test</h1>
+<slot name="a"></slot>
+<slot name="b"></slot>
+<slot name="c"></slot>
+<span>Hello</span>
+
+<style>
+  h1 + span {
+    color: red;
+  }
+</style>