gtest genesis parsing

• only embed chainspec when feature enabled
• change existential deposit to match g1v1
• adapt genesis parsing to real-world data

# build with gtest feature
cargo build --release --features gtest --no-default-features
# run with custom genesis
DUNITER_GTEST_GENESIS=~/dev/py-g1-migrator/output/gtest_genesis.json duniter --tmp --alice --chain=gtest_dev