test: speed up chan/select5

No functional changes.

Generating shorter functions improves compilation time. On my laptop, this test's running time goes from 5.5s to 1.5s; the wall clock time to run all tests goes down 1s. On Raspberry Pi, this CL cuts 50s off the wall clock time to run all tests.

Fixes #7503.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/72590045

test/chan/select5.go

@@ -27,16 +27,16 @@ func main() {
 	fmt.Fprintln(out, header)
 	a := new(arg)
 
-	// Generate each kind of test as a separate function to avoid
+	// Generate each test as a separate function to avoid
 	// hitting the 6g optimizer with one enormous function.
 	// If we name all the functions init we don't have to
 	// maintain a list of which ones to run.
 	do := func(t *template.Template) {
-		fmt.Fprintln(out, `func init() {`)
 		for ; next(); a.reset() {
+			fmt.Fprintln(out, `func init() {`)
 			run(t, a, out)
+			fmt.Fprintln(out, `}`)
 		}
-		fmt.Fprintln(out, `}`)
 	}
 
 	do(recv)