Fix failing postcss integration test (#19436)

This is what's causing the postcss test in #19434 to fail wanted to test
this in isolation without the version bumps

Author: thecrypticace

integrations/postcss/core-as-postcss-plugin.test.ts

@@ -48,7 +48,7 @@ describe.each(Object.keys(variantConfig))('%s', (variant) => {
       },
     },
     async ({ exec, expect }) => {
-      expect(
+      await expect(
         exec('pnpm postcss src/index.css --output dist/out.css', undefined, { ignoreStdErr: true }),
       ).rejects.toThrowError(
         `It looks like you're trying to use \`tailwindcss\` directly as a PostCSS plugin. The PostCSS plugin has moved to a separate package, so to continue using Tailwind CSS with PostCSS you'll need to install \`@tailwindcss/postcss\` and update your PostCSS configuration.`,

integrations/postcss/index.test.ts

@@ -698,9 +698,7 @@ test(
       <div class="flex underline"></div>
     `)
 
-    await process.onStderr((message) =>
-      message.includes('does-not-exist is not exported from package'),
-    )
+    await process.onStderr((message) => message.includes('"./does-not-exist" is not exported'))
 
     expect(await fs.dumpFiles('dist/*.css')).toMatchInlineSnapshot(`
       "
@@ -743,9 +741,7 @@ test(
       `,
     )
 
-    await process.onStderr((message) =>
-      message.includes('does-not-exist is not exported from package'),
-    )
+    await process.onStderr((message) => message.includes('"./does-not-exist" is not exported'))
 
     expect(await fs.dumpFiles('dist/*.css')).toMatchInlineSnapshot(`
       "