patch 8.1.1846: inconsistently using GetVimCommand() and v:progpath

Problem:    Inconsistently using GetVimCommand() and v:progpath. (Daniel
            Hahler)
Solution:   Use GetVimCommand(). (closes #4806)
diff --git a/src/testdir/test_vimscript.vim b/src/testdir/test_vimscript.vim
index cf2c961..69a2eba 100644
--- a/src/testdir/test_vimscript.vim
+++ b/src/testdir/test_vimscript.vim
@@ -2,6 +2,7 @@
 " Most of this was formerly in test49.
 
 source check.vim
+source shared.vim
 
 "-------------------------------------------------------------------------------
 " Test environment							    {{{1
@@ -1721,7 +1722,7 @@
     [CODE]
 
     call writefile(lines, 'Xtest.vim')
-    let res = system(v:progpath .. ' --clean -es -X -S Xtest.vim')
+    let res = system(GetVimCommandClean() .. ' -es -X -S Xtest.vim')
     call assert_equal(0, v:shell_error)
 
     let m = matchstr(res, 'function F1()[^[:print:]]*[[:print:]]*')