libtest terse format: show how far in we are
So for example `./x.py test src/libcore` looks like
```
running 881 tests
.................................................................................................... 100/881
.................................................................................................... 200/881
.................................................................................................... 300/881
.............................................................i.i.................................... 400/881
.................................................................................................... 500/881
.................................................................................................... 600/881
.................................................................................................... 700/881
.................................................................................................... 800/881
.................................................................................
test result: ok. 879 passed; 0 failed; 2 ignored; 0 measured; 0 filtered out
```
When I am waiting for 3500 ui tests to complete, I am often missing some sense of how far in these 3500 it is.
Getting the total count in `write_run_start` is a bit hacky; I did that to not change the "public interface" of the formatters. I can also give them an extra argument in their constructor so that they know from the beginning how many tests there will be. Would you prefer that? (I think I would, but I wanted to get feedback first.)
libtest: Split HumanFormatter into {Pretty,Terse}
libtest: Fixed padding of benchmarks when not benchmarking
libtest: Fixed benchmarks' names not showing in terse-mode
libtest: Formatting
