mental test vs IQ test

mental test and IQ test both are nouns.

Word	Noun	Adjective	Verb	Adverb
mental test	Yes	No	No	No
IQ test	Yes	No	No	No

As nouns, IQ test is a hyponym of mental test; that is, IQ test is a word with a more specific, narrower meaning than mental test:

mental test: any standardized procedure for measuring sensitivity or memory or intelligence or aptitude or personality etc
IQ test: a psychometric test of intelligence

Other hyponyms of mental test include intelligence test, personality test, sub-test.

mental test (noun)	IQ test (noun)
any standardized procedure for measuring sensitivity or memory or intelligence or aptitude or personality etc	a psychometric test of intelligence