flea: put only copymanifest's O_TMPFILE test on tmpfs

Moving the whole module broke the rest of it: Writer::create needs a
runtime directory on a different filesystem from the copy, and with the
test root on /dev/shm none qualifies. Only
a_garbage_stream_falls_back_with_the_tree_untouched opens its manifest
in its own test dir, which the builder's /tmp refuses (EOPNOTSUPP).

pkgbuilds/flea/PKGBUILD

@@ -115,8 +115,12 @@ check() {
   # back to the next create, so on the builder's /tmp the stranger is
   # identical and undo walks it back. tmpfs allocates inode numbers from a
   # counter and never reuses one, which is what the test assumes.
+  # copymanifest's garbage-stream test opens its O_TMPFILE manifest in its
+  # own test dir; the module's other tests must stay off tmpfs, because
+  # Writer::create wants a runtime dir on a different filesystem from the
+  # copy and finds none when both live on /dev/shm.
   local -a filesystem_tests=(
-    backend::copymanifest::tests::
+    backend::copymanifest::tests::a_garbage_stream_falls_back_with_the_tree_untouched
     backend::menu_actions::tests::
     backend::menudelete::tests::
     backend::redo::tests::