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/check/njs.py

@@ -2,5 +2,4 @@ import re
 
 
 def check_njs(output_version):
-    if re.search('--njs', output_version):
-        return True
+    return re.search('--njs', output_version)