patch 8.2.0388: printmbcharset option not tested

Problem:    Printmbcharset option not tested.
Solution:   Add a test.  Enable PostScript for AppVeyor build. (Dominique
            Pelle, closes #5783)
diff --git a/appveyor.yml b/appveyor.yml
index 2d88bed..eb720f4 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -25,9 +25,9 @@
   - cd src/testdir
     # Testing with MSVC gvim
   - path C:\Python35-x64;%PATH%
-  - nmake -f Make_dos.mak VIMPROG=..\gvim
+  - nmake -f Make_dos.mak POSTSCRIPT=yes VIMPROG=..\gvim
   - nmake -f Make_dos.mak clean
     # Testing with MingW console version
-  - nmake -f Make_dos.mak VIMPROG=..\vim
+  - nmake -f Make_dos.mak POSTSCRIPT=yes VIMPROG=..\vim
 
 # vim: sw=2 sts=2 et ts=8 sr