patch 9.0.0034: spell tests do not always clear the word list

Problem:    Spell tests do not always clear the word list.
Solution:   Clear the word list in TearDown(). (closes #10659)
diff --git a/src/testdir/test_spell.vim b/src/testdir/test_spell.vim
index 0187a17..d3f56d8 100644
--- a/src/testdir/test_spell.vim
+++ b/src/testdir/test_spell.vim
@@ -14,6 +14,8 @@
   call delete('Xtest.latin1.add.spl')
   call delete('Xtest.latin1.spl')
   call delete('Xtest.latin1.sug')
+  " set 'encoding' to clear the word list
+  set encoding=utf-8
 endfunc
 
 func Test_wrap_search()
@@ -781,6 +783,10 @@
   set spl=Xtest_ca.latin1.spl
   call assert_equal("elequint", FirstSpellWord())
   call assert_equal("elekwint", SecondSpellWord())
+
+  bwipe!
+  set spellfile=
+  set spl&
 endfunc
 
 func Test_spellfile_value()
@@ -864,9 +870,6 @@
   sil! norm z=
 
   bwipe!
-  " clear the internal word list
-  set enc=latin1
-  set enc=utf-8
 endfunc
 
 func LoadAffAndDic(aff_contents, dic_contents)