Restructure TestCPUProfileMultithreadMagnitude so it will run again with
a longer duration on failure. Log the split between the user vs system
CPU time that rusage reports.
For #50232
Change-Id: Ice5b38ee7594dbee1eaa5686d32b968c306e3e85
Reviewed-on: https://go-review.googlesource.com/c/go/+/393934
Run-TryBot: Rhys Hiltner <rhys@justin.tv>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>