gh-115720: Show number of leaks in huntrleaks progress reports (GH-115726)

Instead of showing a dot for each iteration, show:
- '.' for zero (on negative) leaks
- number of leaks for 1-9
- 'X' if there are more leaks

This allows more rapid iteration: when bisecting, I don't need
to wait for the final report to see if the test still leaks.

Also, show the full result if there are any non-zero entries.
This shows negative entries, for the unfortunate cases where
a reference is created and cleaned up in different runs.

Test *failure* is still determined by the existing heuristic.