test: Add test for new command pci_mps

Adds a test for the new pci_mps command to ensure that it can set the
Maximum Payload Size (MPS) of all devices to 256 bytes in the sandbox
environment. Enables the pci_mps command in the sandbox environment so
that this test can be run.

Signed-off-by: Stephen Carlson <stcarlso@linux.microsoft.com>
diff --git a/test/cmd/Makefile b/test/cmd/Makefile
index 7848f34..055adc6 100644
--- a/test/cmd/Makefile
+++ b/test/cmd/Makefile
@@ -14,6 +14,9 @@
 obj-$(CONFIG_CONSOLE_TRUETYPE) += font.o
 obj-$(CONFIG_CMD_LOADM) += loadm.o
 obj-$(CONFIG_CMD_MEM_SEARCH) += mem_search.o
+ifdef CONFIG_CMD_PCI
+obj-$(CONFIG_CMD_PCI_MPS) += pci_mps.o
+endif
 obj-$(CONFIG_CMD_PINMUX) += pinmux.o
 obj-$(CONFIG_CMD_PWM) += pwm.o
 obj-$(CONFIG_CMD_SEAMA) += seama.o