Does Allah accept the repentance of one who seeks forgiveness for a sin and then returns to committing it? And is success in this worldly life considered a test from Allah?

The swiftness of answered prayers is a good sign. The servant should continuously perform the obligatory acts of worship and increase in supererogatory acts, in gratitude to Allah Almighty for this blessing. This should also motivate him to do good deeds and abandon evil acts, while being cautious not to lose this blessing, as it might be a test. Allah Almighty does not wrong anyone among His creation.