selftests/pcie_bwctrl: Fix test progs list

Commit df6f8c4d ("selftests/pcie_bwctrl: Add 'set_pcie_speed.sh' to
TEST_PROGS") added set_pcie_speed.sh into TEST_PROGS but that script is a
helper that is only being called by set_pcie_cooling_state.sh, not a test
case itself. When set_pcie_speed.sh is in TEST_PROGS, selftest harness will
execute also it leading to bwctrl selftest errors:

  # selftests: pcie_bwctrl: set_pcie_speed.sh
  # cat: /cur_state: No such file or directory
  not ok 2 selftests: pcie_bwctrl: set_pcie_speed.sh # exit=1

Place set_pcie_speed.sh into TEST_FILES instead to have it included into
installed test files but not execute it from the test harness.

Fixes: df6f8c4d ("selftests/pcie_bwctrl: Add 'set_pcie_speed.sh' to TEST_PROGS")
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20250417124529.11391-1-ilpo.jarvinen@linux.intel.com