builder_test: use fonttools' CapturingLogHandler to intercept logger messages
builder_test: use fonttools' CapturingLogHandler to intercept logger messages

It's better to not modify global state (e.g. replacing builder.logger with a dummy logger instance).

The CapturingLogHandler temporarily registers itself as a handler to a logger instance, and clean itself up upon exiting the context manager (even in case of exceptions).

googlei18n/glyphsLib,googlefonts/glyphsLib