tests: use pytest-benchmark for performance tests

pytest-benchmark is a benchmarking suite for individual functions. It
presents itself as a pytest fixture, i.e. you accept it as a test function
argument, and then can use it to benchmark a function.

Output is presented in the following form:

----------------------------------------- benchmark: 1 tests ----------------------------------------
Name (time in s) Min Max Mean StdDev Median IQR Outliers(*) Rounds Iterations
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test_graphmod_git 3.1122 3.1682 3.1295 0.0236 3.1196 0.0313 1;0 5 1
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