tests: allow user to select tests via command line args

[jonasnick]

This PR introduces the ability for users to select specific tests to run via command line arguments. The aim is to eliminate the need to comment out parts of the tests.c file in order to speed up testing. The implementation uses similar command line flags as the benchmarks, for example:

./tests ecmult schnorrsig

The approach taken in this PR may not be the most straightforward, but it ensures backwards compatibility. The shell script located at https://gist.github.com/jonasnick/5e37248e3fa5911cd41e1da2f5f4e395 can be used to test the changes introduced in this PR. I'd be happy to add more flags if needed.