[test_bench] Allow for different names for benchmarks

[andreeaflorescu]

In test_bench, we now expect all benchmarks to be named main. This is because we are running the benchmarks as follows:
cargo bench --bench main --all-features -- --noplot --save-baseline {}.

Instead of harcoding the name to be main, we can parse the Cargo.toml file to check what is the name of the benchmarks.
We need to look for the bench target in Cargo.toml:

[[bench]]
name = "main"

[iaGuoZhi]

Working on it.