selftests/pcie_bwctrl: Add 'set_pcie_speed.sh' to TEST_PROGS

The test shell script "set_pcie_speed.sh" is not installed in INSTALL_PATH.
Attempting to execute set_pcie_cooling_state.sh shows warning:

  ./set_pcie_cooling_state.sh: line 119: ./set_pcie_speed.sh: No such file or directory

Add "set_pcie_speed.sh" to TEST_PROGS.

Link: https://lore.kernel.org/r/Z8FfK8rN30lKzvVV@ly-workstation


Fixes: 838f12c3 ("selftests/pcie_bwctrl: Create selftests")
Signed-off-by: Yi Lai <yi1.lai@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>