pcie_bifurcation: Fetch bifurcation at a PCIe slot with hardcoded values

This will read a json config and return the bifurcation based on that.
It will read the configure file in a persistent file to see if it is
valid machine config and return the bifurcation information
accordingly.

For example,
If it is valid config, it will return 8x8 for PE1,3,4,6.

Tested:
Unit tests passed.

Physical Tests.

PE0 -> no bifurcation
PE1 -> x8x8

```
$ ipmitool raw 0x2e 0x32 0x79 0x2b 0x00 0x0f 0
 79 2b 00 0b 00

$ ipmitool raw 0x2e 0x32 0x79 0x2b 0x00 0x0f 1
 79 2b 00 0b 02 08 08
```

Change-Id: I44aefbfb26372f8bc0069343c8a6d07d3cf6f42d
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/test/handler_mock.hpp b/test/handler_mock.hpp
index 2fb8e2a..9961fa8 100644
--- a/test/handler_mock.hpp
+++ b/test/handler_mock.hpp
@@ -62,6 +62,7 @@
     MOCK_METHOD(void, accelOobWrite,
                 (std::string_view, uint64_t, uint8_t, uint64_t),
                 (const, override));
+    MOCK_METHOD(std::vector<uint8_t>, pcieBifurcation, (uint8_t), (override));
 };
 
 } // namespace ipmi