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.
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 6ea0c45..8ed4642 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -243,6 +243,7 @@
 	test_tagjump \
 	test_taglist \
 	test_tcl \
+	test_termencoding \
 	test_terminal \
 	test_terminal_fail \
 	test_textformat \
@@ -389,6 +390,7 @@
 	test_system.res \
 	test_tab.res \
 	test_tcl.res \
+	test_termencoding.res \
 	test_terminal.res \
 	test_terminal_fail.res \
 	test_textformat.res \
diff --git a/src/testdir/dumps/Test_tenc_euc_jp_01.dump b/src/testdir/dumps/Test_tenc_euc_jp_01.dump
new file mode 100644
index 0000000..14a4313
--- /dev/null
+++ b/src/testdir/dumps/Test_tenc_euc_jp_01.dump
@@ -0,0 +1,10 @@
+>E+0&#ffffff0|8|9|:| |バ*&|ッ|フ|ァ| +&|%|l|d| |の*&|変|更|は|保|存|さ|れ|て|い|ま|せ|ん| +&|(|!| |で*&|変|更|を|破|棄|)+&| @13
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|X+3#0000000&|e|u|c|_|j|p|.|t|x|t| @45|1|,|1| @11|A|l@1
+|E+0&&|8|3|:| |バ*&|ッ|フ|ァ|を|作|成|で|き|な|い|の|で|、|他|の|を|使|用|し|ま|す|.+&@2| @22
+|~+0#4040ff13&| @73
+|~| @73
+|[+1#0000000&|N|o| |N|a|m|e|]| |[|+|]| @43|1|,|5|2| @10|A|l@1
+| +0&&@74
diff --git a/src/testdir/test_termencoding.vim b/src/testdir/test_termencoding.vim
new file mode 100644
index 0000000..6192dcf
--- /dev/null
+++ b/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