Internal: add regression testing (#298)

Author: yordis

src/__tests__/fixtures/regression.css

@@ -0,0 +1,7 @@
+@custom-selector :--button .button;
+
+:--button {
+  & i {
+    color: black;
+  }
+}

src/__tests__/fixtures/regression.expected.css

@@ -0,0 +1,7 @@
+.button {
+}
+
+.button i {
+
+    color: black
+}

src/__tests__/index.js

@@ -1,5 +1,6 @@
 import tape from "tape"
 
+import utils from "./utils"
 import postcssnext from ".."
 
 tape("postcss-cssnext is a postcss plugin", (t) => {
@@ -10,3 +11,19 @@ tape("postcss-cssnext is a postcss plugin", (t) => {
 
   t.end()
 })
+
+tape("cssnext regression test", (t) => {
+  const input = utils.readFixture("regression")
+  const expected = utils.readFixture("regression.expected")
+  const actual = postcssnext().process(input).css.trim()
+
+  utils.write(utils.fixturePath("regression.actual"), actual)
+
+  t.equal(
+    actual,
+    expected.trim(),
+    "should pass the regression"
+  )
+
+  t.end()
+})