[TST] test_deconcatenate_column tests should be consolidated together

[ericmjl]

This is in reference to the last PR by @shandou (#484). I let the two separate tests go to not hold up the PR, but we should put the two separate tests together.

Relevant context:

• Contents of this file should be moved into test_deconcatenate_column.py

[shandou]

Sorry, my bad! As the culprit, I'll take this issue @ericmjl and have it fixed :P

[ericmjl]

@shandou don’t worry about it - I have already put in the PR for this!

[ericmjl]

Closed by #492.