Add test case where %n is passed into a ck_assert call

Add test case where %n is passed into a ck_assert call

In the past it was possible to cause issues if arguments
were passed into ck_assert* macros which included %n.
Exist unit tests check for output format options, such as
%d and %f. No test existed for %n, which writes a value to
memory if used.

Ulrich Eckhardt observed that passing something with a %n in it
would result in issues running Check 0.9.10. Although the
issue is already resolved, this commit adds the test case mentioned in
Ulrich's GitHub issue: https://github.com/libcheck/check/issues/41