[doctest] run with "--help" for options
== TEST CASE ==================================================================
main.cpp(0)
testing the factorial function

main.cpp(0) ERROR!
  CHECK( factorial(0) == 1 )
with expansion:
  CHECK( 0 == 1 )

===============================================================================
[doctest] test cases:    1 |    0 passed |    1 failed |    0 skipped
[doctest] assertions:    5 |    4 passed |    1 failed |