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: fbb8531e58bd ("selftests: extract common functions in ethtool-common.sh")
Signed-off-by: Wang Liang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

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

···
 	udp_tunnel_nic.sh \
 # end of TEST_PROGS
 
+TEST_FILES := \
+	ethtool-common.sh
+# end of TEST_FILES
+
 include ../../../lib.mk