Add regression test for CLI shebang preservation (#1901)

Following up on https://github.com/prettier/prettier/pull/1899

tests_integration/__tests__/__snapshots__/with-shebang.js.snap

@@ -0,0 +1,7 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`preserves shebang 1`] = `
+"#!/usr/bin/env node
+\\"use strict\\";
+"
+`;

tests_integration/__tests__/with-shebang.js

@@ -0,0 +1,9 @@
+"use strict";
+
+const runPrettier = require("../runPrettier");
+
+test("preserves shebang", () => {
+  const result = runPrettier("cli/with-shebang", ["issue1890.js"]);
+
+  expect(result.stdout).toMatchSnapshot();
+});

tests_integration/cli/with-shebang/issue1890.js

@@ -0,0 +1,2 @@
+#!/usr/bin/env node
+"use strict";