test(breakpoint): add unit tests

tests/Breakpoint/BreakpointObserver.test.svelte

@@ -0,0 +1,21 @@
+<script lang="ts">
+  import { breakpointObserver } from "carbon-components-svelte";
+
+  export let smallerThanMd = false;
+  export let largerThanMd = false;
+
+  const observer = breakpointObserver();
+  const smallerThan = observer.smallerThan("md");
+  const largerThan = observer.largerThan("md");
+
+  smallerThan.subscribe((value) => {
+    smallerThanMd = value;
+  });
+
+  largerThan.subscribe((value) => {
+    largerThanMd = value;
+  });
+</script>
+
+<div data-testid="smaller-than-md">{smallerThanMd}</div>
+<div data-testid="larger-than-md">{largerThanMd}</div>