Use "%zd" format specifier for printing number of testcases executed.

Summary:
This helps to avoid signed integer overflow after running a fast fuzz target for several hours, e.g.:

<...>
Done -1097903291 runs in 54001 second(s)



Reviewers: kcc

Reviewed By: kcc

Differential Revision: https://reviews.llvm.org/D29941

llvm-svn: 295112