tests: print an info message after building tests

tests: print an info message after building tests

We can't automatically run unit tests from 'make check' as the test
executable works together with the gpio-mockup kernel module. Without
any additional udev configuration /dev/gpiochipX files are only
accessible for superuser.

Print a message explaining what to do after the tests are built.

NOTE: the message says that linux kernel >=v4.11 is needed to run the
tests. This is not true currently, as we don't test events yet. Event
injecting over debugfs is a new feature of gpio-mockup and will be
available in the upcoming kernel release.

Once v4.11 is released and appropriate test cases implemented, we'll
make it a hard requirement for tests.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>