feat(comp): Allow to inject commandline for tests

By allowing to specify the commandLine for completion, it becomes
possible to have regression tests for fish completion.
These tests are part of the acceptance-testing repo.

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

cmd/helm/completion.go

@@ -304,7 +304,13 @@ end
 function __helm_get_completions
     # Use the cache if possible
     if not set -q __helm_cache_completions
-       set -g __helm_cache_completions (__helm_perform_completion (commandline))
+       if not set -q __helm_comp_command_line
+	      set -g __helm_comp_command_line (commandline)
+	      __helm_debug "Setting commandline to: $__helm_comp_command_line"
+	   end
+
+       set -g __helm_cache_completions (__helm_perform_completion "$__helm_comp_command_line")
+       set -e __helm_comp_command_line
        __helm_debug "Populated completion cache with: $__helm_cache_completions"
     end