Since you can't run pigs, or "squeeges", it will be more difficult.
But maybe the same way you put the water in might work.
You will need some connections to pump the water in and to get it out.
Normally these is done via temporary headers connected to each end of the pipeline, which are removed just before welding the new pipe to the old. See here,
Or you might be able to use some other kind of taps.
One connection to pump water in, located at the lowest point if possible.
You will need another connection, preferably at the high point, to vent air out.
Convention is to run pigs pushed by a dry gas, air or nitrogen, through the pipe and push out all the water, then continue doing that until the content is as dry as you need.
If that is not possible, you may be able to drain water out at the low point after finishing the test and then just blow dry gas through until pipe content is dry. How well or easy it will be to do that depends on how many high and low points there are and the pipe's drainage profile. Obviously having one uniform slope will be easier to drain than a profile with lots of ups and downs.
Has the pipe to be tested been welded to the other pipe yet, or can you still connect ends to test headers?
Can you post the elevation profile of the pipe to be tested?