source the files instead

This *might* be a bit faster running under TSAN, otherwise it takes >
400 seconds on Github Actions.

If this doesn't work we need to disable it for TSAN.

tests/checks/check-completions.fish

@@ -4,7 +4,7 @@
 # No output is good output
 for f in $__fish_data_dir/completions/*.fish
     if type -q (string replace -r '.*/([^/]+).fish' '$1' $f)
-        set -l out ($fish $f 2>&1 | string collect)
+        set -l out (source $f 2>&1 | string collect)
         test -n "$out"
         and echo -- OUTPUT from $f: $out
     end