We should close fd before the return of run_test.

Fixes: 3f2ed813 ("tools: PCI: Add a userspace tool to test PCI endpoint")
Signed-off-by: Hewenliang <hewenliang4@huawei.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>