syscall: remove GOMAXPROCS change in TestExecHelper

TestExec and TestExecHelper check for a workaround of a particular OS
bug on darwin that's triggered more often via asynchronous preemption.

As part of this, the test sets up 100 CPU-bound goroutines, and sets
GOMAXPROCS to 50, sleeping for a little bit before calling Exec. Thus
far, this is fine because the scheduler runs the Execing goroutine in a
timely manner. However, CL 309869 will reduce the minimum heap size,
causing a GC to happen during the test.

On a 16 CPU machine, with GOMAXPROCS at 50, and 100 CPU-bound
goroutines, both the OS scheduler and the Go scheduler are severly
oversaturated. As a result, the test often (not always, but often) runs
out for the full lifetime of those 100 goroutines, which run for about 1
second.

The minimum heap size reduction is not necessary to trigger this; an
additional call to runtime.GC in the helper is also sufficient to
trigger this delay.

The delay on its own isn't great, since it adds a whole second to
all.bash on its own. However, it also seems correlated with other
subprocess tests in the syscall package, namely TestPassFD and
TestFcntlFlock. These tests fail in a fairly superficial way: the file
descriptor for the temporary directories they make gets clobbered, is
closed, or becomes stale.

Change-Id: I213dd5e38967d19a8b317e6d4c5024b57f9e3fed
Reviewed-on: https://go-review.googlesource.com/c/go/+/360574
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/syscall/exec_unix_test.go

@@ -13,7 +13,6 @@ import (
 	"os"
 	"os/exec"
 	"os/signal"
-	"runtime"
 	"syscall"
 	"testing"
 	"time"
@@ -327,7 +326,6 @@ func TestExecHelper(t *testing.T) {
 	// We don't have to worry about restoring these values.
 	// We are in a child process that only runs this test,
 	// and we are going to call syscall.Exec anyhow.
-	runtime.GOMAXPROCS(50)
 	os.Setenv("GO_WANT_HELPER_PROCESS", "3")
 
 	stop := time.Now().Add(time.Second)