attestation-tool no longer runs

[jethrogb]

When running a freshly-built attestation-tool from the master branch, I get the following panic:

thread 'main' panicked at 'Cannot start a runtime from within a runtime. This happens because a function (like `block_on`) attempted to block the current thread while the thread is being used to drive asynchronous tasks.', intel-sgx/enclave-runner/src/usercalls/mod.rs:1037:19
stack backtrace:
...
   2: tokio::runtime::context::runtime::enter_runtime
   3: tokio::runtime::runtime::Runtime::block_on
   4: enclave_runner::usercalls::EnclaveState::run
   5: enclave_runner::usercalls::EnclaveState::library_entry
   6: enclave_runner::library::Library::call
   7: report_test::report
   8: attestation_tool::main::{{closure}}
   9: tokio::runtime::scheduler::current_thread::Context::enter
  10: tokio::runtime::context::set_scheduler
  11: tokio::runtime::scheduler::current_thread::CoreGuard::block_on
  12: tokio::runtime::context::runtime::enter_runtime
  13: tokio::runtime::runtime::Runtime::block_on
  14: attestation_tool::main

Seems related to #534