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feat: streaming debug logfile

This decouples the log file writing from the terminal logging. We now
open an append only file at the start of the process and stream logs to
it. We still only display the log file message in timing mode or if
there is an error, but the file is still written regardless.

All logging now goes through `proc-log` and this is the first step to
removing `npmlog`. For now `npmlog` is still used for the terminal
logging but with a shim in front of it to make it easier to test and
use in conjunction with `proc-log`. Ref: npm/statusboard#366

This also refactors many of the tests to always use an explicit
`t.testdir` for their cache since the file is opened on each `new Npm()`.
Tests are also refactored to use more of `MockNpm` with behavior to
add config items and load `npm` if necessary. A new fixture `mockGlobals`
was also added to make much of this more ergonomic. Ref: npm/statusboard#410

Closes npm/statusboard#411
Closes npm/statusboard#367

PR-URL: https://github.com/npm/cli/pull/4062
Credit: @lukekarrys
Close: #4062
Reviewed-by: @wraithgar

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