tests: escape % before passing string to ck_abort_msg()

tests: escape % before passing string to ck_abort_msg()

ch_abort_msg() expects printf arguments. Passing in a constant
string that happens to have '%' in it will not end as expected,
as the underlying vsnprintf will attempt to format them.

As some unit tests now attempt to have improper output involving
'%' in expressions, it is now possible that the failure message
which is expected during Check's unit tests involve '%'. As such,
only a properly escaped string should be passed to ck_abort_msg().

This change will force the string passed to ck_abort_msg() in the
test_check_failure_msgs unit test to properly escape any '%'
found, so that if a failure did happen the correct text is printed
to the screen.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@1182 64e312b2-a51f-0410-8e61-82d0ca0eb02a