Here's why:
* Infinitive: An infinitive is the base form of a verb, usually preceded by "to" (e.g., to run, to eat, to finish).
* Infinitive Phrase: An infinitive phrase is an infinitive plus any modifiers or complements that follow it.
In this sentence, "to finish" is the infinitive, and "your test" is the object of the infinitive. Together they form the infinitive phrase that acts as the object of the adjective "happy."