When I use cookiecutter, especially in the first phases of the project, I find myself debugging the instantiated copy of the template, finding the problem, and then having to go back to the template to fix it there as well.
This all feels redundant, and I feel that if it was possible to run the template files without physically coping them and instantiating them everytime I change something would make the development much faster.
Is there any way to test the cookiecutter template files without instantiating the project?
Unfortunately you cannot do this otherwise and there is a good reason to be that way: The errors, you may find, may be just errors in your code, but may be caused by the values, provided by the user, or the defaults you provided in the cookiecutter.json, or the way you use those in your templates...
What I do in your situation is the following: I don't template my project at the very beginning. I am writing the code just the way it is without cookiecutter. On the way, I am writing down notes what I want to put in the template. I am marking what should be turned into variable, what should be the default value, variable file and foldernames, etc.
When I am done with my bootstrap and I think it is good enough to fire version 1.0, I start templating it after my notes and then test the template one more time to ensure that I haven't missed something.
Yes, after that point, you will have to evolve the template and you won't be able to avoid this redundant testing, but it is not so intensive as at the beginning.