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/20250219125039.18024-14-shaw.leon@gmail.com

Fixes: 77442ff ("selftests: can: Import tst-filter from can-tests")
Reviewed-by: Vincent Mailhol <mailhol@kernel.org>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Link: https://patch.msgid.link/fa4c0ea262ec529f25e5f5aa9269d84764c67321.1757516009.git.dcaratti@redhat.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Author: dcaratti

tools/testing/selftests/net/can/config

@@ -0,0 +1,3 @@
+CONFIG_CAN=m
+CONFIG_CAN_DEV=m
+CONFIG_CAN_VCAN=m