patch 8.2.1498: on slow systems tests can be flaky

Problem:    On slow systems tests can be flaky.
Solution:   Use TermWait() instead of term-wait(). (Yegappan Lakshmanan,
            closes #6756)
diff --git a/src/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 6ec4ef2..1aebe05 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -2566,7 +2566,7 @@
   call assert_equal(2, winnr())
   let buf = term_start(&shell, #{hidden: 1})
   call popup_create(buf, {})
-  call term_wait(buf, 100)
+  call TermWait(buf, 100)
   call popup_clear(1)
   call assert_equal(2, winnr())