Add a test that runs all available completions

Meaning completions where we have the command.

No completion should be printing anything when sourced.

This could have prevented #8896

tests/checks/check-completions.fish

@@ -0,0 +1,9 @@
+#RUN: %fish -C 'set -l fish %fish' %s
+# Test all completions where the command exists
+
+# No output is good output
+for f in $__fish_data_dir/completions/*.fish
+    if command -q (string replace -r '.*/([^/]+).fish' '$1' $f)
+        $fish $f
+    end
+end