(test): ensure tsdx.config.js w/ rollup-plugin-postcss works

- add and import a basic CSS file in build-withConfig
  - use the existing tsdx.config.js for the rollup config

- ensure that one CSS file gets extracted at the end
- ensure that it is properly autoprefixed and minifed too

test/integration-tests/fixtures/build-withConfig/src/index.css

@@ -0,0 +1,4 @@
+/* ::placeholder should be autoprefixed, and everything minified */
+.test::placeholder {
+  color: 'blue';
+}

test/integration-tests/fixtures/build-withConfig/src/index.ts

@@ -1,3 +1,5 @@
+import './index.css';
+
 export const sum = (a: number, b: number) => {
   if ('development' === process.env.NODE_ENV) {
     console.log('fuck');

test/integration-tests/tsdx-build-withConfig.test.js

@@ -0,0 +1,64 @@
+const shell = require('shelljs');
+const fs = require('fs-extra');
+
+const util = require('../utils/fixture');
+const { execWithCache } = require('../utils/shell');
+
+shell.config.silent = false;
+
+const testDir = 'integration-tests';
+const fixtureName = 'build-withConfig';
+const stageName = `stage-integration-${fixtureName}`;
+
+describe('integration :: tsdx build :: tsdx.config.js', () => {
+  beforeAll(() => {
+    util.teardownStage(stageName);
+    util.setupStageWithFixture(testDir, stageName, fixtureName);
+  });
+
+  it('should create a CSS file in the dist/ directory', () => {
+    const output = execWithCache('node ../dist/index.js build');
+
+    // TODO: this is kind of subpar naming, rollup-plugin-postcss just names it
+    // the same as the output file, but with the .css extension
+    expect(shell.test('-f', 'dist/build-withconfig.cjs.development.css'));
+
+    expect(output.code).toBe(0);
+  });
+
+  it('should autoprefix and minify the CSS file', async () => {
+    const output = execWithCache('node ../dist/index.js build');
+
+    const cssText = await fs.readFile(
+      './dist/build-withconfig.cjs.development.css'
+    );
+
+    // autoprefixed and minifed output
+    expect(
+      cssText.includes('.test::-webkit-input-placeholder{color:"blue"}')
+    ).toBeTruthy();
+
+    expect(output.code).toBe(0);
+  });
+
+  it('should compile files into a dist directory', () => {
+    const output = execWithCache('node ../dist/index.js build');
+
+    expect(shell.test('-f', 'dist/index.js')).toBeTruthy();
+    expect(
+      shell.test('-f', 'dist/build-withconfig.cjs.development.js')
+    ).toBeTruthy();
+    expect(
+      shell.test('-f', 'dist/build-withconfig.cjs.production.min.js')
+    ).toBeTruthy();
+    expect(shell.test('-f', 'dist/build-withconfig.esm.js')).toBeTruthy();
+
+    expect(shell.test('-f', 'dist/index.d.ts')).toBeTruthy();
+
+    expect(output.code).toBe(0);
+  });
+
+  afterAll(() => {
+    util.teardownStage(stageName);
+  });
+});