PCI: Add KUnit tests for __pci_read_base()

Currently it is hard to debug issues in the resource assignment code due
to long reporduction cycles between the developer trying to fix the code
and the user testing it due to the lack of hardware device with the
developer [1].

[1]:
https://lore.kernel.org/all/20210621123714.GA3286648@bjorn-Precision-5520/

This adds KUnit tests for __pci_read_base() which is only  dependent
on software structures, so no hardware is needed to run these.

This lays the foundation for test fixtures we can use to reproduce the
resource assignment code path of PCI.

Sample output from KUnit Test run:

      # Subtest: __pci_read_base()
      1..3
      # test_pci_read_base_type_0_hdr_approach_1: initializing __pci_read_base() tests
   (null): reg 0x18: [mem 0x4f400000-0x4f400fff]
      ok 1 - test_pci_read_base_type_0_hdr_approach_1
      # test_pci_read_base_type_0_hdr_approach_2: initializing __pci_read_base() tests
   (null): reg 0x18: [mem 0x4f400000-0x4f400fff]
   (null): reg 0x1c: [mem 0xaf400000-0xaf4000ff]
      ok 2 - test_pci_read_base_type_0_hdr_approach_2
      # test_pci_read_base_type_1_hdr: initializing __pci_read_base() tests
   (null): reg 0x10: [mem 0xaf400000-0xaf4000ff]
      ok 3 - test_pci_read_base_type_1_hdr
  # __pci_read_base(): pass:3 fail:0 skip:0 total:3
  # Totals: pass:3 fail:0 skip:0 total:3
  # ok 8 - __pci_read_base()

Suggested-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Naveen Naidu <naveennaidu479@gmail.com>

drivers/pci/Kconfig

@@ -252,6 +252,10 @@ config PCIE_BUS_PEER2PEER
 
 endchoice
 
+config PCI_READ_BASE_KUNIT_TEST
+	tristate "KUnit tests for __pci_read_base() in probe.c"
+	depends on PCI && KUNIT=y
+
 source "drivers/pci/hotplug/Kconfig"
 source "drivers/pci/controller/Kconfig"
 source "drivers/pci/endpoint/Kconfig"

drivers/pci/Makefile

@@ -36,4 +36,7 @@ obj-$(CONFIG_PCI_ENDPOINT)	+= endpoint/
 obj-y				+= controller/
 obj-y				+= switch/
 
+# KUnit test files
+obj-$(CONFIG_PCI_READ_BASE_KUNIT_TEST) += pci-read-base-test.o
+
 subdir-ccflags-$(CONFIG_PCI_DEBUG) := -DDEBUG