sub-test vs mental test

sub-test and mental test both are nouns.

Word	Noun	Adjective	Verb	Adverb
sub-test	Yes	No	No	No
mental test	Yes	No	No	No

As nouns, mental test is a hypernym of sub-test; that is, mental test is a word with a broader meaning than sub-test:

sub-test: one of a battery of related tests
mental test: any standardized procedure for measuring sensitivity or memory or intelligence or aptitude or personality etc

Other hypernyms of sub-test include mental testing, psychometric test, test.

sub-test (noun)	mental test (noun)
one of a battery of related tests	any standardized procedure for measuring sensitivity or memory or intelligence or aptitude or personality etc