patch 9.1.1051: tests: no support for env variables when running Vim in terminal

Problem:  tests: no support for env variables when running Vim in
          terminal
Solution: support the "env" argument in RunVimInTerminal(),
          close swapfiles properly in test_termcodes,
          use CheckFeature in test_termencoding

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_termencoding.vim b/src/testdir/test_termencoding.vim
index de812ce..9b7f487 100644
--- a/src/testdir/test_termencoding.vim
+++ b/src/testdir/test_termencoding.vim
@@ -1,10 +1,10 @@
 " Test for setting 'encoding' to something else than the terminal uses, then
 " setting 'termencoding' to make it work.
 
+source check.vim
+
 " This only works with "iconv".
-if !has('iconv')
-  throw 'Skipped: iconv feature missing'
-endif
+CheckFeature iconv
 
 source screendump.vim
 if !CanRunVimInTerminal()