Try this one (it's the output from the generator in https://www.hackerrank.com/challenges/find-the-nearest-clone/forum/comments/592337 ):

http://etotheipiplusone.com/projects/hackerrank/find-the-nearest-clone-testcase-large-0.txt

11MB download; 1'000'000 nodes and 1'000'000 edges, so obeys the problem constraints.

It should be easy to generate similarly tough, but less "predictable" ones, too; in fact, here's another, slightly crazier one:

http://etotheipiplusone.com/projects/hackerrank/find-the-nearest-clone-testcase-large-09.txt

or

http://etotheipiplusone.com/projects/hackerrank/find-the-nearest-clone-testcase-large-14.txt

This one is the most evil one I've created so far :)

http://vps2.etotheipiplusone.com:30176/public_html/hackerrank/find-the-nearest-clone-testcase-large-15.txt