Allow test to have arbitrary resources

[vkarak]

The way we handle this currently is very monolithic. A test can only have a single sourcesdir which will be copied over to its stage directory. Also there is the system-level resourcesdir option that defines global system directory that can be used inside a test.

Accepting a single sourcesdir is problematic as it prohibits plugins (aka mixins) from defining their resources as the could potentially "steal" those defined in the test that they are supposed to augment. So the plugins must copy their resource files manually and not through reframe.

The fact that sourcesdir is a directory is also problematic as a test can "pull" resources from different places.

We should revise how test resources are handled and make them more flexible. Since this can be a breaking change, I'm adding it to 5.0.

[vkarak]

Another thing related to resources that should be addressed, is that those are copied too late, practically after any pre-compile or pre-run hook (depending on the test). This makes it impossible to modify the test based on a copied resource and any modification to a resource has to be done through prebuild_cmds or prerun_cmds.