Faster testing and benchmarking

[mgree]

We have a lot of sleep calls that could be faster.

Write a tool to detect when a mount is ready. (Current benchmarks do this by busy looping on umount.) Use its exit status to determine when it's okay to proceed on the mount and proceed checking that process has completed.

It'll also be useful to have a way to pad a few milliseconds on, i.e., unmount means the process may not be done yet. The current utils/timeout script is okay, but (a) it's noisy, and (b) it'd be nice to not fix it on ffs when we start. Maybe a waitfor command that takes a time limit before killing the process?

[mgree]

One possibility: spin on the presence of an xattr on the root directory. The attribute will appear as soon as the mount is live.