patch 8.1.2129: using hard coded executable path in test

Problem:    Using hard coded executable path in test.
Solution:   Use v:progpath. Use $VIMRUNTIME instead of "runtime". (James
            McCoy, closes #5025)
diff --git a/src/testdir/test_spell.vim b/src/testdir/test_spell.vim
index a44d955..5231266 100644
--- a/src/testdir/test_spell.vim
+++ b/src/testdir/test_spell.vim
@@ -130,18 +130,18 @@
   new
 
   set enc=latin1 spell spelllang=en
-  call assert_match("^\nfile: .*/runtime/spell/en.latin1.spl\n$", execute('spellinfo'))
+  call assert_match("^\nfile: " .. $VIMRUNTIME .. "/spell/en.latin1.spl\n$", execute('spellinfo'))
 
   set enc=cp1250 spell spelllang=en
-  call assert_match("^\nfile: .*/runtime/spell/en.ascii.spl\n$", execute('spellinfo'))
+  call assert_match("^\nfile: " .. $VIMRUNTIME .. "/spell/en.ascii.spl\n$", execute('spellinfo'))
 
   set enc=utf-8 spell spelllang=en
-  call assert_match("^\nfile: .*/runtime/spell/en.utf-8.spl\n$", execute('spellinfo'))
+  call assert_match("^\nfile: " .. $VIMRUNTIME .. "/spell/en.utf-8.spl\n$", execute('spellinfo'))
 
   set enc=latin1 spell spelllang=en_us,en_nz
   call assert_match("^\n" .
-                 \  "file: .*/runtime/spell/en.latin1.spl\n" .
-                 \  "file: .*/runtime/spell/en.latin1.spl\n$", execute('spellinfo'))
+                 \  "file: " .. $VIMRUNTIME .. "/spell/en.latin1.spl\n" .
+                 \  "file: " .. $VIMRUNTIME .. "/spell/en.latin1.spl\n$", execute('spellinfo'))
 
   set spell spelllang=
   call assert_fails('spellinfo', 'E756:')