test: alias-completion: Improve rm test by using function and not alias

Works better, and more sane

test/plugins/alias-completion.plugin.bats

@@ -9,6 +9,13 @@ cite _about _param _example _group _author _version
 
 load ../../completion/available/capistrano.completion
 
+teardown() {
+  # Mostly for the hidden behind function test
+  unset -f rm
+
+  rm -f test.file
+}
+
 @test "alias-completion: See that aliases with double quotes and brackets do not break the plugin" {
   alias gtest="git log --graph --pretty=format:'%C(bold)%h%Creset%C(magenta)%d%Creset %s %C(yellow)<%an> %C(cyan)(%cr)%Creset' --abbrev-commit --date=relative"
   load ../../plugins/available/alias-completion.plugin
@@ -23,9 +30,13 @@ load ../../completion/available/capistrano.completion
   assert_success
 }
 
-@test "alias-completion: See that having aliased rm command does not output unnecessary output" {
-  alias rm='rm -v'
+@test "alias-completion: See that hidden behind function rm command does not change behavior" {
+  rm() {
+    touch test.file;
+    command rm "$@";
+  }
   load ../../plugins/available/alias-completion.plugin
 
-  refute_output
+  run stat test.file
+  assert_failure
 }