debug/gosym: do not forget to close test binay file handle in TestPCLine

Fixes #15121

Change-Id: I651521743c56244c55eda5762905889d7e06887a
Reviewed-on: https://go-review.googlesource.com/21521
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

gosym/pclntab_test.go

@@ -208,6 +208,7 @@ func TestPCLine(t *testing.T) {
 	defer endtest()
 
 	f, tab := crack(pclinetestBinary, t)
+	defer f.Close()
 	text := f.Section(".text")
 	textdat, err := text.Data()
 	if err != nil {