internal/trace/v2: make TestGCStress less random

Currently TestGCStress' main loop allocates a randomly-sized byte slice
in a loop. On the windows-386 builder, it looks like the following is
happening.

In such heavily-allocating scenarios, the test seems to be able to
outrun the GC. This is a known issue on all platforms, but it looks to
me like there may be a real issue with mark termination. (Fixing that is
outside the scope of this change, but relevant here.)

Furthermore, while the test is ramping up, the pacer is taking time to
acclimate to the high allocation rate. This is probably made worse due
to the coarse time granularity on Windows, since the pacer relies on
accurate time measurements.

Because the pacer is ramping up, it isn't starting early enough, causing
a lot of memory to get allocated black and inflate the live heap size.
This happens for more than one cycle.

Last but not least, because the core allocating loop of this test
allocates randomly-sized byte slices, we could just get unlucky and
inflate the live heap by much more sometimes. Furthermore, the
randomness creates chaos for the pacer that is totally unnecessary for
this test.

Although I couldn't reproduce the issue we're seeing on the trybots in a
gomote, I *could* reproduce memory spikes in general. These memory
spikes always occurred before the pacer had a chance to "warm up," in
the first two cycles after the heavy allocating begins.

I believe the flakiness we're seeing is all of these factors lining up,
because if I just make the size of the allocated byte slices smaller and
non-random, I can no longer reproduce the memory spikes. This change
implements this as a fix in the hope that it'll resolve the flakiness.

Fixes #66624.

Change-Id: I478d45e7c600e5aee4b21dbe831e1f287284f5e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/578319
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/internal/trace/v2/testdata/testprog/gc-stress.go

@@ -11,7 +11,6 @@ package main
 
 import (
 	"log"
-	"math/rand"
 	"os"
 	"runtime"
 	"runtime/trace"
@@ -62,7 +61,7 @@ func main() {
 		i := i
 		go func() {
 			for {
-				sink[i] = make([]byte, rand.Intn(32<<10))
+				sink[i] = make([]byte, 4<<10)
 			}
 		}()
 	}