selftests: netdevsim: Fix ethtool-coalesce.sh fail by installing ethtool-common.sh

The script "ethtool-common.sh" is not installed in INSTALL_PATH, and
triggers some errors when I try to run the test
'drivers/net/netdevsim/ethtool-coalesce.sh':

  TAP version 13
  1..1
  # timeout set to 600
  # selftests: drivers/net/netdevsim: ethtool-coalesce.sh
  # ./ethtool-coalesce.sh: line 4: ethtool-common.sh: No such file or directory
  # ./ethtool-coalesce.sh: line 25: make_netdev: command not found
  # ethtool: bad command line argument(s)
  # ./ethtool-coalesce.sh: line 124: check: command not found
  # ./ethtool-coalesce.sh: line 126: [: -eq: unary operator expected
  # FAILED /0 checks
  not ok 1 selftests: drivers/net/netdevsim: ethtool-coalesce.sh # exit=1

Install this file to avoid this error. After this patch:

  TAP version 13
  1..1
  # timeout set to 600
  # selftests: drivers/net/netdevsim: ethtool-coalesce.sh
  # PASSED all 22 checks
  ok 1 selftests: drivers/net/netdevsim: ethtool-coalesce.sh

Fixes: fbb8531e58 ("selftests: extract common functions in ethtool-common.sh")
Signed-off-by: Wang Liang <wangliang74@huawei.com>
Link: https://patch.msgid.link/20251030040340.3258110-1-wangliang74@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/netdevsim/Makefile

@@ -20,4 +20,8 @@ TEST_PROGS := \
 	udp_tunnel_nic.sh \
 # end of TEST_PROGS
 
+TEST_FILES := \
+	ethtool-common.sh
+# end of TEST_FILES
+
 include ../../../lib.mk