tests/test_font.py: test_4457(): update to match updated mupdf 1.26.x.

Cherr-pick of MuPDF master commit 05df7a13234aca146bc341a6240cd367732c8265 `Bug
708660: Fix mistake in renumbering objects when saving.` gives 100% correct
behaviour, with rms_after==0.

Author: julian-smith-artifex-com

tests/test_font.py

@@ -318,7 +318,7 @@ def test_4457():
         assert text_before == text
         assert rms_before == 0
 
-        if pymupdf.mupdf_version_tuple >= (1, 27):
+        if pymupdf.mupdf_version_tuple >= (1, 26, 6):
             assert rms_after == 0
         else:
             # As of 2025-05-20 there are some differences in some characters,