support cargo test

Is there any chance we can make `cargo test` work with [our enclave code](https://github.com/scs/substraTEE-worker/tree/master/enclave)?

it would be great if we could use standard `cargo test`. Right now we can only do [handmade testing](https://github.com/scs/substraTEE-worker/blob/master/worker/src/tests/mod.rs) which is a lot less convenient to track

right now we get a clash between `std` and `sgx_tstd`:

```
Compiling substratee-worker-enclave v0.6.0 (/home/abrenzikofer/substraTEE-worker/enclave)
error: duplicate lang item in crate `std` (which `test` depends on): `f32_runtime`.
|
= note: the lang item is first defined in crate `sgx_tstd` (which `substratee_worker_enclave` depends on)

error: duplicate lang item in crate `std` (which `test` depends on): `f64_runtime`.
|
= note: the lang item is first defined in crate `sgx_tstd` (which `substratee_worker_enclave` depends on)

error: duplicate lang item in crate `std` (which `test` depends on): `panic_impl`.
|
= note: the lang item is first defined in crate `sgx_tstd` (which `substratee_worker_enclave` depends on)

error: duplicate lang item in crate `std` (which `test` depends on): `begin_panic`.
|
= note: the lang item is first defined in crate `sgx_tstd` (which `substratee_worker_enclave` depends on)

error: duplicate lang item in crate `std` (which `test` depends on): `oom`.
|
= note: the lang item is first defined in crate `sgx_tstd` (which `substratee_worker_enclave` depends on)

error: aborting due to 5 previous errors

error: could not compile `substratee-worker-enclave`.

```

Start with the enclave directory linked in the issue and compare its current setup with worker/src/tests/mod.rs, which contains the handmade tests. Run cargo test in the enclave and investigate the reported std and sgx_tstd duplicate lang-item errors. Done means the enclave tests run through standard cargo test without those conflicts.

Written by the indexing model from the issue text.