testing: introduce util function to synchronize concurrent commands on files
This is an extension of mechanisms that the tests have been using for a while.
To be able to also control the execution in Rust, we introduce utility to
perform such `wait_on_file` logic based on some configuration value.
This will be used in the tests introduced in the next changesets.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true