test: close file in goFiles

Brad's battery died on a plane and the file stayed open for 8 years
without anyone noticing. 😄

Someone noticed in https://github.com/gopherjs/gopherjs/pull/950.

Updates #2833

Change-Id: I46b28ac014a8c355be94e026615f119f96e5d51a
Reviewed-on: https://go-review.googlesource.com/c/go/+/218700
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

test/run.go

@@ -203,6 +203,7 @@ func goFiles(dir string) []string {
 	f, err := os.Open(dir)
 	check(err)
 	dirnames, err := f.Readdirnames(-1)
+	f.Close()
 	check(err)
 	names := []string{}
 	for _, name := range dirnames {