cmake, test: Introduce `SECP256K1_TESTNAME_PREFIX` variable

[hebasto]

This change enables downstream projects, such as Bitcoin Core, to add a custom prefix to the libsecp256k1 tests, facilitating the use of ctest regex matching options.

For instance, after configuring with -DSECP256K1_TESTNAME_PREFIX="secp256k1_", a downstream project users can filter their tests like that:

ctest --tests-regex "secp256k1"

or

ctest --exclude-regex "secp256k1"

[hebasto]

cc @fanquake @theuni