print list of supported scales for each test class

[smoors]

ReFrame sometimes doesn't run the test cases you want to run after filtering, which can be annoying to debug.

to help with this, i've added some printing/logging for the scales.

EDIT: i've managed to find a way to do it in the mixin class, so no changes are needed in the test classes

Scales supported by EESSI_NumPy: ('1_core', '2_cores', '4_cores', '1_8_node', '1_4_node', '1_2_node', '1_node')

EDIT2: i've now changed it so that it only prints if --verbose is set

[casparvl]

Lgtm!