selftests: pci_endpoint: Migrate to Kselftest framework

selftests: pci_endpoint: Migrate to Kselftest framework

Migrate the PCI endpoint test to Kselftest framework. All the tests that
were part of the previous pcitest.sh file were migrated.

Below is the list of tests converted:

   1. BAR0 Test
   2. BAR1 Test
   3. BAR2 Test
   4. BAR3 Test
   5. BAR4 Test
   6. BAR5 Test
   7. Consecutive BAR Tests
   8. Legacy IRQ Tests
   9. MSI Interrupt Tests (MSI1 to MSI32)
  10. MSI-X Interrupt Tests (MSI-X1 to MSI-X2048)
  11. Read Tests - MEMCPY (For 1, 1024, 1025, 1024000, 1024001 Bytes)
  12. Write Tests - MEMCPY (For 1, 1024, 1025, 1024000, 1024001 Bytes)
  13. Copy Tests - MEMCPY (For 1, 1024, 1025, 1024000, 1024001 Bytes)
  14. Read Tests - DMA (For 1, 1024, 1025, 1024000, 1024001 Bytes)
  15. Write Tests - DMA (For 1, 1024, 1025, 1024000, 1024001 Bytes)
  16. Copy Tests - DMA (For 1, 1024, 1025, 1024000, 1024001 Bytes)

BAR, DMA and MEMCPY tests are added as fixture variants and can be executed
separately as below:

  $ pci_endpoint_test -v BAR0
  $ pci_endpoint_test -v dma
  $ pci_endpoint_test -v memcpy

Link: https://lore.kernel.org/r/20250116171650.33585-5-manivannan.sadhasivam@linaro.org
Co-developed-by: Aman Gupta <aman1.gupta@samsung.com>
Co-developed-by: Padmanabhan Rajanbabu <p.rajanbabu@samsung.com>
[mani: reworked based on the IOCTL fix, cleanups, documentation, commit message]
Signed-off-by: Aman Gupta <aman1.gupta@samsung.com>
Signed-off-by: Padmanabhan Rajanbabu <p.rajanbabu@samsung.com>
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>