prepare gtest runtime

Based on !168 (merged). Prepares deployment of ĞTest network with chainspecs made of custom genesis and easily customizable client specs (bootnodes...).

Build with

cargo build --features gtest --no-default-features

Run with

# reads genesis from file given in environment variable or generate one if not given
DUNITER_GTEST_GENESIS=./node/specs/gtest_genesis.json ./target/debug/duniter --tmp --alice --chain=gtest_de
# reads genesis and client spec from ./node/specs/gtest_*.json at runtime
./target/debug/duniter --tmp --alice --chain=gtest_live
# hardcoded genesis and client spec from ./node/specs/gtest_*.json at compile time
./target/debug/duniter --tmp --alice --chain=gtest