runtime/pprof: disable GC during calibration test

TestCPUProfileMultithreadMagnitude compares Go's CPU profile against the
OS's accounting of the process's execution time, expecting them to be
near equal. Background work from the runtime (especially in the garbage
collector) can add significant noise to that measurement and flakiness
to the test. Disable automatic GC cycles during the test.

Updates #49065

Change-Id: Ie88895bfea17374278c5187f3a83e9f486bd37fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/359934


Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>