unix: modernize test helpers

Test helper functions mktmpfifo and chtmpdir were written
before t.Cleanup was available, which necessitated returning
of a cleanup function and the use of defer.

Let's use t.Cleanup (available since Go 1.14) and simplify the callers.

While at it,
 - use t.Helper (added in Go 1.9);
 - simplify some error messages (errors that come from os package
   usually doesn't need to be wrapped).

Change-Id: Id981ae1c85fa2642a76358a31fc18a9af2f78711
Reviewed-on: https://go-review.googlesource.com/c/sys/+/526695
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

unix/syscall_openbsd_test.go

@@ -12,9 +12,8 @@ import (
 )
 
 func TestPpoll(t *testing.T) {
-	defer chtmpdir(t)()
-	f, cleanup := mktmpfifo(t)
-	defer cleanup()
+	chtmpdir(t)
+	f := mktmpfifo(t)
 
 	const timeout = 100 * time.Millisecond