feat(test): Adapt completion tests to Cobra 1.1

Cobra 1.1 trims completions so we need to remove extra spaces from
the tests.

Signed-off-by: Marc Khouzam <marc.khouzam@montreal.ca>

cmd/helm/testdata/output/plugin_args_comp.txt

@@ -1,6 +1,6 @@
 plugin.complete was called
 Namespace: default
 Num args received: 1
-Args received: 
+Args received:
 :4
 Completion ended with directive: ShellCompDirectiveNoFileComp

cmd/helm/testdata/output/plugin_args_flag_comp.txt

@@ -1,6 +1,6 @@
 plugin.complete was called
 Namespace: default
 Num args received: 2
-Args received: --myflag 
+Args received: --myflag
 :4
 Completion ended with directive: ShellCompDirectiveNoFileComp

cmd/helm/testdata/output/plugin_args_ns_comp.txt

@@ -1,6 +1,6 @@
 plugin.complete was called
 Namespace: mynamespace
 Num args received: 1
-Args received: 
+Args received:
 :2
 Completion ended with directive: ShellCompDirectiveNoSpace

cmd/helm/testdata/output/plugin_echo_bad_directive.txt

@@ -1,6 +1,6 @@
 echo plugin.complete was called
 Namespace: default
 Num args received: 1
-Args received: 
+Args received:
 :0
 Completion ended with directive: ShellCompDirectiveDefault

cmd/helm/testdata/output/plugin_echo_no_directive.txt

@@ -1,6 +1,6 @@
 echo plugin.complete was called
 Namespace: mynamespace
 Num args received: 1
-Args received: 
+Args received:
 :0
 Completion ended with directive: ShellCompDirectiveDefault