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/tls.py

@@ -6,7 +6,6 @@ def check_openssl(output_version):
     try:
         subprocess.check_output(['which', 'openssl'])
     except subprocess.CalledProcessError:
-        return None
+        return False
 
-    if re.search('--openssl', output_version):
-        return True
+    return re.search('--openssl', output_version)