[libra_fuzzer] disable test target for fuzz_runner
[libra_fuzzer] disable test target for fuzz_runner

Testing for fuzz targets is covered by `cargo test` in `testsuite/libra_fuzzer`.

Closes #624.

aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra