Add assignment pattern tests.

Reviewed by @tolmasky.

Author: tolmasky

test/compare-acorn-es6/assignment-pattern.expected.js

@@ -0,0 +1,2 @@
+function x(a = { a: 6 }, b = null, ...d) {
+}

test/compare-acorn-es6/assignment-pattern.expected.min.js

@@ -0,0 +1 @@
+function x(a={a:6},b=null,...d){}

test/compare-acorn-es6/assignment-pattern.js

@@ -0,0 +1,3 @@
+function x(a={a:6}, b=null, ...d)
+{
+}