Fixed instant finalization of genesis block on light client

[svyatonik]

reported by @rphmeier

I'm having test failure on this branch, as well as on master:

test tests::panic_execution_with_foreign_code_gives_error ... FAILED
test tests::successful_execution_with_native_equivalent_code_gives_ok ... ok
test tests::deploying_wasm_contract_should_work ... ok
test tests::full_wasm_block_import_works ... ok
test tests::wasm_big_block_import_fails ... ok

failures:

---- tests::panic_execution_with_foreign_code_gives_error stdout ----
thread 'tests::panic_execution_with_foreign_code_gives_error' panicked at 'assertion failed: `(left == right)`
  left: `Ok(Success)`,
 right: `Err(CantPay)`', node/executor/src/lib.rs:139:3
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:211
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:221
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:491
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:398
   6: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:353
   7: node_executor::tests::panic_execution_with_foreign_code_gives_error
   8: <F as alloc::boxed::FnBox<A>>::call_box
             at src/libtest/lib.rs:1471
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/ops/function.rs:238
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/liballoc/boxed.rs:673
   9: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102


failures:
    tests::panic_execution_with_foreign_code_gives_error

, but I'm unsure whether the reason is my laptop, or master branch (since #1781 ). So I'll wait until CI is failed/passed before fixing it.

[rphmeier]

CI has passed, so the test failure is either spurious or local