With this mod, you can freely test out various fights. You can select units or spawnlists (the system used by the game to generate parties), specifying their numbers or strength. You can select the battlefield, the map, if you want fortifications or some special music, whether you want your bros to join or you'd like to control your allies, among other things.
There's also a screen that allows you to add units to an ongoing fight, if it is a simulated fight.

This mod is still unfinished, so expect bugs. It might interfere with normal gameplay, too. It is probably best to only add this mod to data when you want to actually simulate fights, and remove it during normal gameplay.
Feel free to suggest further content.

How to install:
-download the file and place it into \Battle Brothers\data\ without unzipping.
-you'll need modding script hooks v19+ and MSU

How to use:
Press ctrl+S while on the worldmap.
The actions should be mostly self-explanatory. "Copy" and "Paste" setup work via the clipbar. Upon pressing "Copy" or pressing "Start Battle", a string (text) containing the current setup will be copied. You can paste this in a text file. To load it again, simply copy that text, and press the "Paste" button.
While in a custom battle, you can also bring up the spawning menu with ctrl+S. Drag and drop onto the field to spawn units. It will be assigned to the active faction.
Most UI elements are labeled, also check the keybinds modsettings to see more functionality.

Thanks to Luftwaffle for the nifty icons top right.