add 'show subscriber' command, alias for 'subscriber ... show'

When I wrote the osmo-hlr subscriber command, I failed to heed the common
'show foo' scheme and instead created a 'subscriber [...] show' command.
Relieve that weirdness by creating an alias that has 'show' at the start.

Arrange string macros so that the 'show subscriber' cmd doesn't end in a space
(the SUBSCR macro ends in a space ' ' to implicitly include the space to
commands like 'create', 'show', 'update').

Add the new command to test_nodes.vty and test_subscriber.vty.

Change-Id: I01ce9b0868302d40ed05c6a588316a194d6071e4
diff --git a/tests/test_nodes.vty b/tests/test_nodes.vty
index a9d4ac4..e1f57a9 100644
--- a/tests/test_nodes.vty
+++ b/tests/test_nodes.vty
@@ -18,6 +18,7 @@
   show talloc-context (application|all) (full|brief|DEPTH) filter REGEXP
   show gsup-connections
   subscriber (imsi|msisdn|id) IDENT show
+  show subscriber (imsi|msisdn|id) IDENT
 
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