selftests: can: enable CONFIG_CAN_VCAN as a module

A proper kernel configuration for running kselftest can be obtained with:

$ yes | make kselftest-merge

Build of 'vcan' driver is currently missing, while the other required knobs
are already there because of net/link_netns.py [1]. Add a config file in
selftests/net/can to store the minimum set of kconfig needed for CAN
selftests.

[1] https://patch.msgid.link/[e-mail]

Fixes: 77442ffa83e8 ("selftests: can: Import tst-filter from can-tests")
Reviewed-by: Vincent Mailhol <[e-mail]>
Signed-off-by: Davide Caratti <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

tools/testing/selftests/net/can/config

···
+CONFIG_CAN=m
+CONFIG_CAN_DEV=m
+CONFIG_CAN_VCAN=m