[cmake] Add comments to express what tests do.

Summary: The current way of having an echo command before running the test is rather pointless as the echo is only rendered once the test finishes. Presumably, the author of these echo command intended to use the COMMENT directive.

Test Plan:

ninja check-bitcoin-util
ninja check-rpcauth
ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4536