patch 8.1.1144: too strict checking of the 'spellfile' option

Problem:    Too strict checking of the 'spellfile' option.
Solution:   Allow for a path.
diff --git a/src/testdir/test_spell.vim b/src/testdir/test_spell.vim
index 6bf43ac..b3143c9 100644
--- a/src/testdir/test_spell.vim
+++ b/src/testdir/test_spell.vim
@@ -376,6 +376,11 @@
   call assert_equal("elekwint", SecondSpellWord())
 endfunc
 
+func Test_spellfile_value()
+  set spellfile=Xdir/Xtest.latin1.add
+  set spellfile=Xdir/Xtest.utf-8.add,Xtest_other.add
+endfunc
+
 func Test_region_error()
   messages clear
   call writefile(["/regions=usgbnz", "elequint/0"], "Xtest.latin1.add")