patch 8.2.4982: colors in terminal window are not 100% correct

Problem:    Colors in terminal window are not 100% correct.
Solution:   Use g:terminal_ansi_colors as documented. (closes #10429,
            closes #7227 closes #10347)

src/testdir/test_functions.vim

@@ -2679,8 +2679,8 @@ func Test_range()
     else
       let cmd = "ls"
     endif
-    call assert_fails('call term_start("' .. cmd .. '", #{term_finish: "close"})', 'E475:')
-    unlet g:terminal_ansi_colors
+    call assert_fails('call term_start("' .. cmd .. '", #{term_finish: "close"'
+        \ .. ', ansi_colors: range(16)})', 'E475:')
   endif
 
   " type()

src/testdir/test_terminal.vim

@@ -2012,10 +2012,16 @@ func Test_terminal_ansicolors_global()
   CheckFeature termguicolors
   CheckFunction term_getansicolors
 
+  if has('vtp') && !has('vcon') && !has('gui_running')
+    throw 'Skipped: does not support termguicolors'
+  endif
+
+  set tgc
   let g:terminal_ansi_colors = reverse(copy(s:test_colors))
   let buf = Run_shell_in_terminal({})
   call assert_equal(g:terminal_ansi_colors, term_getansicolors(buf))
   call StopShellInTerminal(buf)
+  set tgc&
 
   exe buf . 'bwipe'
   unlet g:terminal_ansi_colors
@@ -2025,6 +2031,11 @@ func Test_terminal_ansicolors_func()
   CheckFeature termguicolors
   CheckFunction term_getansicolors
 
+  if has('vtp') && !has('vcon') && !has('gui_running')
+    throw 'Skipped: does not support termguicolors'
+  endif
+
+  set tgc
   let g:terminal_ansi_colors = reverse(copy(s:test_colors))
   let buf = Run_shell_in_terminal({'ansi_colors': s:test_colors})
   call assert_equal(s:test_colors, term_getansicolors(buf))
@@ -2047,6 +2058,7 @@ func Test_terminal_ansicolors_func()
   let colors[4] = 'Invalid'
   call assert_fails('call term_setansicolors(buf, colors)', 'E254:')
   call assert_fails('call term_setansicolors(buf, {})', 'E714:')
+  set tgc&
 
   call StopShellInTerminal(buf)
   call assert_equal(0, term_setansicolors(buf, []))