[CMAKE] Rename secp256k1 test targets

Summary:
Running the secp256k1 tests is obvious, using a check-secp256k1-*
target. But building the target without running the tests is much more
complicated, since names like tests doesn't tell much.
This diff renames the targets to prefix with secp256k1-* just like we
do with bitcoin-*.

Test Plan:

ninja secp256k1-tests
ninja secp256k1-exhaustive_tests
ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6313