run-tests.py: Rewrite checker.py

* Reuse mmap tags - each mmap call will result in a new <mapped-addrX> tag.
* Trim diff output if it exceeds 20 lines.
* Gracefully exit on SIGSEGV.
* Add checker flags:
   - verbose, to display the test output
   - diff, to display differences from the ref file
   - memcheck, test solution for memory leaks with valgrind

Signed-off-by: Alex Apostolescu <alexx.apostolescu@gmail.com>