patch 8.2.0448: various functions not properly tested

Problem:    Various functions not properly tested.
Solution:   Add more tests, especially for failures. (Yegappan Lakshmanan,
            closes #5843)

src/testdir/test_exists.vim

@@ -68,6 +68,10 @@ func Test_exists()
   " Existing environment variable
   let $EDITOR_NAME = 'Vim Editor'
   call assert_equal(1, exists('$EDITOR_NAME'))
+  if has('unix')
+    " ${name} environment variables are supported only on Unix-like systems
+    call assert_equal(1, exists('${VIM}'))
+  endif
   " Non-existing environment variable
   call assert_equal(0, exists('$NON_ENV_VAR'))
 
@@ -323,3 +327,5 @@ endfunc
 func Test_exists_funcarg()
   call FuncArg_Tests("arg1", "arg2")
 endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab