The symbol_unicode table has 130 entries; only U+F0DE was covered by a test.
Add symbol_unicode_table_covers_the_adobe_encoding, which asserts every
code->real-Unicode mapping directly against the authoritative
unicode.org/MAPPINGS/VENDORS/ADOBE/symbol.txt table, plus that ASCII
pass-through and CUS fragment codes return None. It is pure and
platform-independent (no fonts involved), so it runs on every machine and
catches any typo in the table.