[SPARK-8681] fixed wrong ordering of columns in crosstab

I specifically randomized the test. What crosstab does is equivalent to a countByKey, therefore if this test fails again for any reason, we will know that we hit a corner case or something.

cc rxin marmbrus

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #7060 from brkyvz/crosstab-fixes and squashes the following commits:

0a65234 [Burak Yavuz] addressed comments v1
d96da7e [Burak Yavuz] fixed wrong ordering of columns in crosstab