tests: Check return code instead of ouput for does_library_support

There can be some noise in the output (like python deprecation
warnings).

tests/scripts/does_library_support.py

@@ -59,4 +59,5 @@ def is_supported(var):
         print("Usage: python %s <parameter>" % sys.argv[0])
         sys.exit(-1)
 
-    print(is_supported(sys.argv[1]))
+    ret = is_supported(sys.argv[1])
+    sys.exit(0) if ret else sys.exit(1)

tests/test-verify-pool-members.yml

@@ -78,14 +78,15 @@
     executable: "{{ ansible_python.executable }}"
   register: grow_supported
   changed_when: false
+  failed_when: grow_supported.rc not in [0, 1]
 
 - name: Verify that PVs fill the whole devices when they should
   include_tasks: verify-pool-member-pvsize.yml
   loop: "{{ _storage_test_pool_pvs | default([]) }}"
   loop_control:
     loop_var: st_pool_pv
   when:
-    - grow_supported.stdout | trim == 'True'
+    - grow_supported.rc == 0
     - storage_test_pool.type == "lvm"
     - storage_test_pool.grow_to_fill | bool