tests: consolidate CR removal/addition functions

tests: consolidate CR removal/addition functions

append_cr(), remove_cr(), q_to_nul() and q_to_cr() are defined in multiple
tests. Consolidate them into test-lib.sh so we can stop redefining them.
The use of remove_cr() in t0020 to test for a CR is replaced with a new
function has_cr() to accurately reflect what is intended (the output of
remove_cr() was being thrown away).

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>