ReFrame should not try loading files that do not have a registered test

[vkarak]

ReFrame will load a Python file as a test if it has a "reframe" import or the @simple_test decorator:

reframe/reframe/frontend/loader.py

Line 38 in deb6107

return self._has_import or self._has_regression_test

I think this is wrong, because in a recursive load with -R, the loader may pick up utility or library files that are meant to be imported by others tests and not directly, causing weird warnings. ReFrame should only load files that actually want to register tests.