The engine would have to be tested in a wind-tunnel due to the airspeed startup requirements.
Rather than running the ramjet on the ground, I think it's also equally likely the improvements were part of the actual Dornier 17 aerial testing.

I think the other quote talking about the p13b prototype construction at various facilities most likely refers to the DM1 itself rather than a ramjet engine fitted p13.
When I return home from interstate I have a book devoted to the p13 and DM1 which documents the construction of the DM1 in detail. I'll post the results then.
Subscale versions of the p13 were definitely wind-kanal tested in flight configuration with the engine intake tube rather than just in DM1 deltawing configuration. Interestingly one of the photographs of the p13 undergoing string-tuft tests shows forward canard planes affixed to the nose intake. I imagine Lippisch was unsure about pitch behaviour and control during landing or flying near stall speed and added those as insurance. Once the deltas low speed behaviour was discovered he would have realised they were not necessary. The Eurofighter, Kfir, Griffon and Viggen were all deltas with forward canards.
Ordinary Deltas are of course rather forgiving at lowspeed and can fly approach with that remarkable nose up high orientation with the wing supported by enormous bound leading edge vortices.