Don't save/restore font-backend in framesets (bug#38442)

* lisp/frameset.el (frameset-persistent-filter-alist):
Add :never filter for `font-backend'.
1 job for master in 57 minutes (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #4409
test-all

00:57:00

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:319: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1