cmake, test: Add `secp256k1_` prefix to test names

[hebasto]

This PR improves regex matching options when using ctest in downstream projects, such as Bitcoin Core.

For instance, a downstream project users can filter their tests like that:

ctest --tests-regex "secp256k1"

or

ctest --exclude-regex "secp256k1"

A ctest log with this PR:

$ ctest --test-dir build -j 16
Internal ctest changing into directory: /home/hebasto/git/secp256k1/secp256k1/build
Test project /home/hebasto/git/secp256k1/secp256k1/build
    Start 1: secp256k1_noverify_tests
    Start 2: secp256k1_tests
    Start 3: secp256k1_exhaustive_tests
    Start 4: secp256k1_ecdsa_example
    Start 5: secp256k1_ecdh_example
    Start 6: secp256k1_schnorr_example
    Start 7: secp256k1_ellswift_example
    Start 8: secp256k1_musig_example
1/8 Test #4: secp256k1_ecdsa_example ..........   Passed    0.00 sec
2/8 Test #5: secp256k1_ecdh_example ...........   Passed    0.00 sec
3/8 Test #6: secp256k1_schnorr_example ........   Passed    0.00 sec
4/8 Test #7: secp256k1_ellswift_example .......   Passed    0.00 sec
5/8 Test #8: secp256k1_musig_example ..........   Passed    0.00 sec
6/8 Test #3: secp256k1_exhaustive_tests .......   Passed    6.19 sec
7/8 Test #1: secp256k1_noverify_tests .........   Passed   38.83 sec
8/8 Test #2: secp256k1_tests ..................   Passed   91.66 sec

100% tests passed, 0 tests failed out of 8

Total Test time (real) =  91.67 sec