runtime: refactor the scavenger and make it testable
    
    This change refactors the scavenger into a type whose methods represent
    the actual function and scheduling of the scavenger. It also stubs out
    access to global state in order to make it testable.
    
    This change thus also adds a test for the scavenger. In writing this
    test, I discovered the lack of a behavior I expected: if the
    pageAlloc.scavenge returns < the bytes requested scavenged, that means
    the heap is exhausted. This has been true this whole time, but was not
    documented or explicitly relied upon. This change rectifies that. In
    theory this means the scavenger could spin in run() indefinitely (as
    happened in the test) if shouldStop never told it to stop. In practice,
    shouldStop fires long before the heap is exhausted, but for future
    changes it may be important. At the very least it's good to be
    intentional about these things.
    
    While we're here, I also moved the call to stopTimer out of wake and
    into sleep. There's no reason to add more operations to a context that's
    already precarious (running without a P on sysmon).
    
    Change-Id: Ib31b86379fd9df84f25ae282734437afc540da5c
    Reviewed-on: https://go-review.googlesource.com/c/go/+/384734
    Reviewed-by: Michael Pratt <mpratt@google.com>
    Run-TryBot: Michael Knyszek <mknyszek@google.com>
    TryBot-Result: Gopher Robot <gobot@golang.org>