Add more tests for face :extend attribute

* test/lisp/faces-tests.el (faces--test-extend-with-themes):
New set of tests for verifying the :extend attribute across
theme loading and unloading, and also via face inheritance.
1 job for master in 55 minutes and 31 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #4464


Name Stage Failure
test-all Test
make[2]: *** [check-doit] Error 1
make[1]: *** [check-expensive] Error 2
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]: 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