[cmake] Make bench-secp256k1 actually run benchmarks, not simply build them.

Summary: Also ensures that all benchmarks are run in the terminal thread. Thsi ensures that no two benchmark run at the same time, making the result meaningless, and ensure that we get the result dumped on the terminal as the benchmark runs.

Test Plan:

ninja bench-secp256k1
ninja bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4832