rcutorture: Summarize summary of build and run results

When running the default list of tests, the run summary of a successful
(that is, failed to find any errors) run fits easily on a 24-line screen.
But a run with something like "--configs '5*CFLIST'" will be 80 lines long,
and it is all too easy to miss a failure message when scrolling back.
This commit therefore prints out the number of runs with failing builds
or runtime failures, but only if there are any such failures.

For example, a run with a single build error and a single runtime error
would print two lines like this:

1 runs with build errors.
1 runs with runtime errors.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>