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_system.vim b/src/testdir/test_system.vim
index f09f509..deee51f 100644
--- a/src/testdir/test_system.vim
+++ b/src/testdir/test_system.vim
@@ -1,5 +1,7 @@
 " Tests for system() and systemlist()
 
+source shared.vim
+
 func Test_System()
   if !executable('echo') || !executable('cat') || !executable('wc')
     return
@@ -49,11 +51,11 @@
 
 func Test_system_exmode()
   if has('unix') " echo $? only works on Unix
-    let cmd = ' -es -u NONE -c "source Xscript" +q; echo "result=$?"'
+    let cmd = ' -es -c "source Xscript" +q; echo "result=$?"'
     " Need to put this in a script, "catch" isn't found after an unknown
     " function.
     call writefile(['try', 'call doesnotexist()', 'catch', 'endtry'], 'Xscript')
-    let a = system(v:progpath . cmd)
+    let a = system(GetVimCommand() . cmd)
     call assert_match('result=0', a)
     call assert_equal(0, v:shell_error)
   endif
@@ -61,32 +63,32 @@
   " Error before try does set error flag.
   call writefile(['call nosuchfunction()', 'try', 'call doesnotexist()', 'catch', 'endtry'], 'Xscript')
   if has('unix') " echo $? only works on Unix
-    let a = system(v:progpath . cmd)
+    let a = system(GetVimCommand() . cmd)
     call assert_notequal('0', a[0])
   endif
 
-  let cmd = ' -es -u NONE -c "source Xscript" +q'
-  let a = system(v:progpath . cmd)
+  let cmd = ' -es -c "source Xscript" +q'
+  let a = system(GetVimCommand() . cmd)
   call assert_notequal(0, v:shell_error)
   call delete('Xscript')
 
   if has('unix') " echo $? only works on Unix
-    let cmd = ' -es -u NONE -c "call doesnotexist()" +q; echo $?'
-    let a = system(v:progpath. cmd)
+    let cmd = ' -es -c "call doesnotexist()" +q; echo $?'
+    let a = system(GetVimCommand() . cmd)
     call assert_notequal(0, a[0])
   endif
 
-  let cmd = ' -es -u NONE -c "call doesnotexist()" +q'
-  let a = system(v:progpath. cmd)
+  let cmd = ' -es -c "call doesnotexist()" +q'
+  let a = system(GetVimCommand(). cmd)
   call assert_notequal(0, v:shell_error)
 
   if has('unix') " echo $? only works on Unix
-    let cmd = ' -es -u NONE -c "call doesnotexist()|let a=1" +q; echo $?'
-    let a = system(v:progpath. cmd)
+    let cmd = ' -es -c "call doesnotexist()|let a=1" +q; echo $?'
+    let a = system(GetVimCommand() . cmd)
     call assert_notequal(0, a[0])
   endif
 
-  let cmd = ' -es -u NONE -c "call doesnotexist()|let a=1" +q'
-  let a = system(v:progpath. cmd)
+  let cmd = ' -es -c "call doesnotexist()|let a=1" +q'
+  let a = system(GetVimCommand() . cmd)
   call assert_notequal(0, v:shell_error)
 endfunc