os/exec: fix number of expected file descriptors on Plan 9

Since CL 104570043 and 112720043, we are using the
nsec system call instead of /dev/bintime on Plan 9.

LGTM=rsc
R=rsc
CC=aram, golang-codereviews
https://golang.org/cl/155590043

src/os/exec/exec_test.go

@@ -258,15 +258,7 @@ var testedAlreadyLeaked = false
 // basefds returns the number of expected file descriptors
 // to be present in a process at start.
 func basefds() uintptr {
-	n := os.Stderr.Fd() + 1
-
-	// Go runtime for 32-bit Plan 9 requires that /dev/bintime
-	// be kept open.
-	// See ../../runtime/time_plan9_386.c:/^runtime·nanotime
-	if runtime.GOOS == "plan9" && runtime.GOARCH == "386" {
-		n++
-	}
-	return n
+	return os.Stderr.Fd() + 1
 }
 
 func closeUnexpectedFds(t *testing.T, m string) {