tests: debugging, benchmarks and profiling

[teknico]

This does five things:

• adds the PuDB interactive debugger and a script to run it on tests;
• adds the pytest-benchmark plugin and rework the database benchmark test to use it;
• adds the pytest-profiling plugin (add the --profile or --profile-svg options when running pytest);
• converts unit/crypto/secp256k1/test_field.py to pytest format;
• adds benchmarks for crypto/secp256k1/field.py.