Add substring-based font-lock test assertions

Extend the font-lock test infrastructure with substring-based face
checks alongside the existing positional (start end face) form.
Tests can now use ("defn" font-lock-keyword-face) instead of opaque
numeric positions like (2 5 font-lock-keyword-face), making them
easier to read and resilient to string changes.

Convert the majority of font-lock tests to substring form where it
improves readability. Positional form is preserved for tests involving
escape sequences, compound faces, various-faces, and complex
colon/underscore patterns where substrings would be ambiguous.

Author: bbatsov