selftests: Move PCI Endpoint tests from tools/pci to Kselftests

selftests: Move PCI Endpoint tests from tools/pci to Kselftests

This just moves the existing tests under tools/pci to
tools/testing/selftests/pci_endpoint and adjusts the paths in Makefile
accordingly. Migration to Kselftest framework will be done in subsequent
commits.

Link: https://lore.kernel.org/r/20250116171650.33585-4-manivannan.sadhasivam@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Niklas Cassel <cassel@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>