kcsan: Rename test.c to selftest.c

Rename 'test.c' to 'selftest.c' to better reflect its purpose (Kconfig
variable and code inside already match this). This is to avoid confusion
with the test suite module in 'kcsan-test.c'.

No functional change.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>