VIM commands to set up the files.

# From bash...
$  vim print_integer.h

# From Vim
:split test_print_integer.c

# Handy shortcuts (snippets), optional

# Start a new C file with a main.
i  # insert mode
newc«TAB»

# Insert a new #include "….h"
Inc«TAB»
 
:vs expected.txt

# Click top panel.
:vs print_integer.c

# copied the function declaration from the header file (print_integer.h) to the
# implementation file (test_print_integer.c) and changed the ; to {…}.