stubtest does not correctly handle `disable_error_code` option when `--mypy-config-file` is supplied

[sobolevn]

When --mypy-config-file is passed to stubtest is uses parse_config_file function. Which does not handle disable_error_code setting correctly. Since it is done in another place (mypy/main.py):

mypy/mypy/main.py

Lines 1339 to 1353 in a0dbbd5

	# Process `--enable-error-code` and `--disable-error-code` flags
	disabled_codes = set(options.disable_error_code)
	enabled_codes = set(options.enable_error_code)
	valid_error_codes = set(error_codes.keys())
	invalid_codes = (enabled_codes | disabled_codes) - valid_error_codes
	if invalid_codes:
	parser.error(f"Invalid error code(s): {', '.join(sorted(invalid_codes))}")
	options.disabled_error_codes |= {error_codes[code] for code in disabled_codes}
	options.enabled_error_codes |= {error_codes[code] for code in enabled_codes}
	# Enabling an error code always overrides disabling
	options.disabled_error_codes -= options.enabled_error_codes

There are similar cases of options that are post-processed there, but not in parse_config_file, but let's fix them one by one to reduce potential regressions.

I propose adding process_error_codes method to Options object so it can be used in both main.py and stubtest.py (and others like stubgen in the future).

Found while working on python/typeshed#12463