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

[hebasto]

cc @fanquake @theuni

[real-or-random]

Concept ACK if this helps Core

[sipa]

Alternatively, could we just add a secp256k1_ prefix to our test names?

[hebasto]

Rebased. @sipa's suggestion has been accepted.

[real-or-random]

utACK 87384f5

[theuni]

Concept ACK.

Shouldn't autotools get the same treatment though?

[hebasto]

Shouldn't autotools get the same treatment though?

This PR modifies only test names, while leaving executable names untouched. The concept of "named tests" is not applicable to Autotools. Changing the executable names seems quite invasive though.

[theuni]

Thanks, I misunderstood. Based on sipa's comment I thought this was actually renaming, which would've been fine with me too. That explains the simplicity :)

utACK 87384f5