chore(vrl): test the VM functions as well in the `test_function` macro. (#11546)

* Test the VM functions as well in the test_function macro.

Signed-off-by: Stephen Wakely <fungus.humungus@gmail.com>

* Added comment.

Signed-off-by: Stephen Wakely <fungus.humungus@gmail.com>

* Spelling.

Signed-off-by: Stephen Wakely <fungus.humungus@gmail.com>

* Feedback from Pablo.

Signed-off-by: Stephen Wakely <fungus.humungus@gmail.com>

Co-authored-by: Jesse Szwedko <jesse@szwedko.me>