added output= to new test 1035.013

[mattdowle]

Follow up to #4723
One of its new tests had this output from test.data.table() :

Running test id 1035.013      Duplicate column names found in molten data.table. Setting unique names using 'make.names'

So added output= to the test to capture and test that cat() happening. Other tests have output= for the same cat() too.
The cat() itself seems to be long-standing (maybe it should be prefixed with if (verbose)).

[codecov]

Codecov Report

Merging #4983 (3ba676a) into master (f5c6526) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4983   +/-   ##
=======================================
  Coverage   99.45%   99.45%           
=======================================
  Files          73       73           
  Lines       14612    14612           
=======================================
  Hits        14532    14532           
  Misses         80       80           

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