patch 8.1.0760: no proper test for using 'termencoding'

Problem:    No proper test for using 'termencoding'.
Solution:   Add a screendump test.  Fix using double width characters in a
            screendump.

src/testdir/test_termencoding.vim

@@ -0,0 +1,37 @@
+" Test for setting 'encoding' to something else than the terminal uses, then
+" setting 'termencoding' to make it work.
+
+" This only works with "iconv".
+if !has('multi_byte') || !has('iconv')
+  finish
+endif
+
+source screendump.vim
+if !CanRunVimInTerminal()
+  finish
+endif
+
+" This Vim is running with 'encoding' "utf-8", the Vim in the terminal is
+" running with 'encoding' "euc-jp".  We need to make sure the text is in the
+" right encoding, this is a bit tricky.
+func Test_termencoding_euc_jp()
+  new
+  call setline(1, 'E89: バッファ %ld の変更は保存されていません (! で変更を破棄)')
+  write ++enc=euc-jp Xeuc_jp.txt
+  quit
+
+  call writefile([
+	\ 'set encoding=euc-jp',
+	\ 'set termencoding=utf-8',
+	\ 'scriptencoding utf-8',
+	\ 'exe "normal aE83: バッファを作成できないので、他のを使用します...\<Esc>"',
+	\ 'split Xeuc_jp.txt',
+	\ ], 'XTest_tenc_euc_jp')
+  let buf = RunVimInTerminal('-S XTest_tenc_euc_jp', {'rows': 10})
+  call VerifyScreenDump(buf, 'Test_tenc_euc_jp_01', {})
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete('Xeuc_jp.txt')
+  call delete('XTest_tenc_euc_jp')
+endfunc