Tests: prerequisites checking reworked.

Prerequisites check moved to the module level to simplify class structure.
Discovery and prerequisites checks functions moved to the separate files.
Introduced "require" fixture to provide per-test requirements check.

test/unit/option.py

@@ -4,6 +4,7 @@ import platform
 class Options:
     _options = {
         'architecture': platform.architecture()[0],
+        'available': {'modules': {}, 'features': {}},
         'is_privileged': os.geteuid() == 0,
         'skip_alerts': [],
         'skip_sanitizer': False,