Added Minimal Tests for plugin-postcss (#1357)

* initial commit

* removed unnecessary mocks

* removed snapshots & mocked execa

* removed console logs

plugins/plugin-postcss/test/plugin.test.js

@@ -0,0 +1,46 @@
+const fs = require('fs');
+const path = require('path');
+const plugin = require("../plugin.js");
+
+const cssPath = path.resolve(__dirname, './stubs/style.css');
+const cssContent = fs.readFileSync(cssPath, 'utf-8');
+const configFilePath = path.resolve(__dirname, './stubs/postcss.config.js');
+
+jest.mock('execa')
+const execa = require('execa');
+
+describe('@snowpack/plugin-postcss', () => {
+    beforeEach(() => {
+      execa.mockClear();
+      execaFn = jest.fn(() => {
+            return {
+                stdout: "stdout",
+                stderr: "stderr"
+            }
+      });
+      execa.mockImplementation(execaFn)
+    });
+
+    test('with no options', async () => {
+        const pluginInstance = plugin({}, {});
+        const transformCSSResults = await pluginInstance.transform({
+            fileExt: path.extname(cssPath),
+            contents: cssContent 
+        });
+        expect(execa.mock.calls[0]).toContain('postcss')
+        expect(execa.mock.calls[0][1]).not.toEqual([`--config ${configFilePath}`])
+    });
+
+    test('passing in a config file', async () => {
+        const options = {
+            config: path.resolve(configFilePath)
+        }
+        const pluginInstance = plugin({}, options)
+        const transformCSSResults = await pluginInstance.transform({
+            fileExt: path.extname(cssPath),
+            contents: cssContent 
+        });
+        expect(execa.mock.calls[0]).toContain('postcss')
+        expect(execa.mock.calls[0][1]).toEqual([`--config ${configFilePath}`])
+    });
+});

plugins/plugin-postcss/test/stubs/postcss.config.js

@@ -0,0 +1,7 @@
+module.exports = {
+    plugins: [
+        require('cssnano')({
+            preset: 'default',
+        }),
+    ],
+};

plugins/plugin-postcss/test/stubs/style.css

@@ -0,0 +1,16 @@
+body {
+    background: #fff;
+}
+
+#root {
+    width: 480px;
+    margin: auto;
+    padding: 16px;
+    box-sizing: border-box;
+}
+
+.content {
+    color: blue;
+}
+
+