Update unit tests to support Ubuntu 2020_04 LTS (ARMmbed#2545)

Test AMI's are updated to use Ubuntu 2020-04 LTS and updated tools
require return value to test methods. Add missing return statement
to test cases where return was missing.

Co-authored-by: Arto Kinnunen <arto.kinnunen@pelion.com>

test/nanostack/unittest/6lp_mac/beacon_handler/test_beacon_handler.c

@@ -244,6 +244,7 @@ bool test_mac_beacon_link_beacon_compare_rx_callback_set()
 
     protocol_core_stub.entry_ptr = NULL;
 
+    return true;
 }
 
 bool test_beacon_join_priority_update()
@@ -371,16 +372,14 @@ bool test_beacon_join_priority_update()
     }
     free(entry.mac_parameters->mac_beacon_payload);
 
-
-
     // Reset stub values
     protocol_core_stub.entry_ptr = NULL;
     protocol_6lowpan_stub.uint8_value = 0;
     mac_helper_stub.enable_storing = false;
     mac_helper_stub.uint8_ptr = 0;
     mac_helper_stub.uint8_value = 0;
 
-
+    return true;
 }
 
 

test/nanostack/unittest/6lp_mac/mac_helper/test_mac_helper.c

@@ -383,6 +383,7 @@ bool test_mac_helper_security_pairwisekey_set()
     }
 
     //<-- test mac_helper_keytable_pairwise_descriptor_set here
+    return true;
 }
 
 bool test_mac_helper_security_next_key_set()