misc/cgo/testshared: call flag.Parse in TestMain

Otherwise, some test flags don't work.

Change-Id: Iacf3930d0eec28e4d690cd382adbb2ecf866a0e2
Reviewed-on: https://go-review.googlesource.com/55615Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testshared/shared_test.go

@@ -10,6 +10,7 @@ import (
 	"debug/elf"
 	"encoding/binary"
 	"errors"
+	"flag"
 	"fmt"
 	"go/build"
 	"io"
@@ -161,6 +162,8 @@ func testMain(m *testing.M) (int, error) {
 }
 
 func TestMain(m *testing.M) {
+	flag.Parse()
+
 	// Some of the tests install binaries into a custom GOPATH.
 	// That won't work if GOBIN is set.
 	os.Unsetenv("GOBIN")