patch 8.1.2255: ":term ++shell" does not work on MS-Windows

Problem:    ":term ++shell" does not work on MS-Windows.
Solution:   Add MS-Windows support.
diff --git a/src/testdir/test_terminal.vim b/src/testdir/test_terminal.vim
index 1a126b9..ba17959 100644
--- a/src/testdir/test_terminal.vim
+++ b/src/testdir/test_terminal.vim
@@ -2215,15 +2215,25 @@
 endfunc
 
 func Test_terminal_shell_option()
-  CheckUnix
-  " exec is a shell builtin command, should fail without a shell.
-  term exec ls runtest.vim
-  call WaitForAssert({-> assert_match('job failed', term_getline(bufnr(), 1))})
-  bwipe!
+  if has('unix')
+    " exec is a shell builtin command, should fail without a shell.
+    term exec ls runtest.vim
+    call WaitForAssert({-> assert_match('job failed', term_getline(bufnr(), 1))})
+    bwipe!
 
-  term ++shell exec ls runtest.vim
-  call WaitForAssert({-> assert_match('runtest.vim', term_getline(bufnr(), 1))})
-  bwipe!
+    term ++shell exec ls runtest.vim
+    call WaitForAssert({-> assert_match('runtest.vim', term_getline(bufnr(), 1))})
+    bwipe!
+  elseif has('win32')
+    " dir is a shell builtin command, should fail without a shell.
+    term dir /b runtest.vim
+    call WaitForAssert({-> assert_match('job failed', term_getline(bufnr(), 1))})
+    bwipe!
+
+    term ++shell dir /b runtest.vim
+    call WaitForAssert({-> assert_match('runtest.vim', term_getline(bufnr(), 1))})
+    bwipe!
+  endif
 endfunc
 
 func Test_terminal_setapi_and_call()