tests/fd: Error out early if more fds are open

This is simply an error in test setup. There's a limit to how far we
can isolate them from the system.

(it's possible new cmake versions close fds automatically since I
can't reproduce the original issue via `ninja test` or `make test`)

Fixes #9017

tests/checks/fds.fish

@@ -2,6 +2,14 @@
 
 # Check that we don't leave stray FDs.
 
+set -l fds ($helper print_fds)
+test "$fds" = "0 1 2"
+or begin
+    echo This test needs to have all fds other than 0 1 and 2 closed before running >&2
+    echo Please close the other fds and try again >&2
+    exit 1
+end
+
 $helper print_fds
 # CHECK: 0 1 2