patch 8.1.0711: test files still use function!

Problem:    Test files still use function!.
Solution:   Remove the exclamation mark.  Fix overwriting a function.

src/testdir/test_system.vim

@@ -1,6 +1,6 @@
 " Tests for system() and systemlist()
 
-function! Test_System()
+func Test_System()
   if !executable('echo') || !executable('cat') || !executable('wc')
     return
   endif
@@ -45,9 +45,9 @@ function! Test_System()
   bwipe!
 
   call assert_fails('call system("wc -l", 99999)', 'E86:')
-endfunction
+endfunc
 
-function! Test_system_exmode()
+func Test_system_exmode()
   if has('unix') " echo $? only works on Unix
     let cmd = ' -es -u NONE -c "source Xscript" +q; echo "result=$?"'
     " Need to put this in a script, "catch" isn't found after an unknown