gh-111929: Fix regrtest --pgo: test_str => test_unicode (#111938)

test_unicode was renamed to test_str in Python 3.13, but Python 3.12
still uses test_unicode name.

Author: vstinner

Lib/test/libregrtest/pgo.py

@@ -42,10 +42,10 @@
     'test_set',
     'test_sqlite3',
     'test_statistics',
-    'test_str',
     'test_struct',
     'test_tabnanny',
     'test_time',
+    'test_unicode',
     'test_xml_etree',
     'test_xml_etree_c',
 ]