selftests: pci_endpoint: Add BAR subrange mapping test case

selftests: pci_endpoint: Add BAR subrange mapping test case

Add BAR_SUBRANGE_TEST to the pci_endpoint kselftest suite.

The test uses the PCITEST_BAR_SUBRANGE ioctl and will skip when the
chosen BAR is disabled (-ENODATA), when the endpoint/controller does not
support subrange mapping (-EOPNOTSUPP), or when the BAR is reserved for
the test register space (-EBUSY).

Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260124145012.2794108-9-den@valinux.co.jp