patch 8.1.1426: no test for syntax highlight in popup window
Problem: No test for syntax highlight in popup window.
Solution: Add a screenshot test. Update associated documentation. Avoid
'buftype' being reset by setbufvar().
diff --git a/src/testdir/dumps/Test_popupwin_11.dump b/src/testdir/dumps/Test_popupwin_11.dump
new file mode 100644
index 0000000..21460dd
--- /dev/null
+++ b/src/testdir/dumps/Test_popupwin_11.dump
@@ -0,0 +1,10 @@
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