In cc98f98, logging was changed to use a new 'write_out!' macro that
internally used 'write!' instead of 'print!' to log. This had an
unfortunate side-effect: 'libtest' via 'cargo test' no longer captures
the log output of tests.
The reason this occurs is due to the way that Cargo, or rather
`libtest`, captures log output: it uses hidden, unstable functions that
replace a special sink that `print!`, and _only_ `print!` writes to.
Using `write!` directly, as the commit does, bypasses this sink, and so
`cargo` never captures the output.
As a compromise, we only use the better implementation when we're not
compiled with `debug_assertions` or running tests, so at least tests run
in debug-mode won't spew output.
8573b6069f