[sublime bindings] Add test for smartBackspace

Issue codemirror#4038

Author: marijnh

test/sublime_test.js

@@ -274,6 +274,10 @@
          "clearBookmarks",
          Pos(0, 0), "selectBookmarks", at(0, 0));
 
+  stTest("smartBackspace", "  foo\n    bar",
+         setSel(0, 2, 0, 2, 1, 4, 1, 4, 1, 6, 1, 6), "smartBackspace",
+         val("foo\n  br"))
+
   stTest("upAndDowncaseAtCursor", "abc\ndef  x\nghI",
          setSel(0, 1, 0, 3,
                 1, 1, 1, 1,