add all of the testing

src/plugins/__tests__/postcss.test.ts

@@ -0,0 +1,50 @@
+import { expect, test } from "bun:test";
+import PostCSS from "../postcss";
+
+// Dummy PostCSS plugin that uppercases all CSS
+function uppercasePlugin() {
+  return {
+    postcssPlugin: "uppercase",
+    Once(root: any) {
+      root.walkDecls((decl: any) => {
+        decl.value = decl.value.toUpperCase();
+        decl.prop = decl.prop.toUpperCase();
+      });
+    }
+  };
+}
+uppercasePlugin.postcss = true;
+
+test("PostCSS applies plugins to CSS", async () => {
+  const plugin = new PostCSS([uppercasePlugin()]);
+  const input = "body { color: red; }";
+  const output = await plugin.rewriteFile(input);
+  expect(output).toContain("COLOR: RED;");
+});
+
+test("PostCSS returns unchanged CSS if no plugins", async () => {
+  const plugin = new PostCSS([]);
+  const input = "body { color: blue; }";
+  const output = await plugin.rewriteFile(input);
+  expect(output).toBe(input);
+});
+
+test("PostCSS handles multiple plugins", async () => {
+  // Plugin to replace 'red' with 'green'
+  function replaceRedPlugin() {
+    return {
+      postcssPlugin: "replace-red",
+      Once(root: any) {
+        root.walkDecls((decl: any) => {
+          decl.value = decl.value.replace(/red/g, "green");
+        });
+      }
+    };
+  }
+  replaceRedPlugin.postcss = true;
+
+  const plugin = new PostCSS([replaceRedPlugin(), uppercasePlugin()]);
+  const input = "body { color: red; }";
+  const output = await plugin.rewriteFile(input);
+  expect(output).toContain("COLOR: GREEN;");
+});