patch 8.2.1607: Vim9: getchar() test fails on MS-Windows

Problem:    Vim9: getchar() test fails on MS-Windows.
Solution:   First consume any available input.
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index dcc8087..a80767a 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -1506,6 +1506,8 @@
 enddef
 
 def Test_getchar()
+  while getchar(0)
+  endwhile
   assert_equal(0, getchar(true))
 enddef