|The following is based on developer test content cut from Fallout 4 and has not been confirmed by canon sources.|
CTEST is an unused cell in the game files of Fallout 4. It is marked as "Quick Test Cell" in the local map, a name shared by several other cells.
The cell consists of a sole room, furnished with a mattress, bed and weapons workbench.
The only way to exit the cell is to use a console command such as coc SanctuaryExt to teleport out.
CTEST appears in the game files of Fallout 4 as a test cell.