Clear findfont cache when calling addfont().

... as new fonts may invalidate previous searches (e.g. a previous
findfont may have chosen to return an approximate match, whereas the
newly added font is an exact match).

No test, as there is no machinery to *remove* a font from FontManager
yet so we add a font just temporarily within the test.

lib/matplotlib/font_manager.py

@@ -1092,6 +1092,7 @@ def addfont(self, path):
             font = ft2font.FT2Font(path)
             prop = ttfFontProperty(font)
             self.ttflist.append(prop)
+        self._findfont_cached.__func__.cache_clear()
 
     @property
     def defaultFont(self):