test-lib: Fix say_color () not to interpret \a\b\c in the message

test-lib: Fix say_color () not to interpret \a\b\c in the message

When running with color disabled (e.g. under prove to produce TAP
output), say_color() helper function is defined to use echo to show
the message.  With a message that ends with "\c", echo is allowed to
interpret it as "Do not end the line with LF".

Use printf "%s\n" to emit the message literally.

Signed-off-by: Junio C Hamano <gitster@pobox.com>