System Test: shouldn't need to recompile the example contracts once for each test

[paulbellamy]

What problem does your feature solve?

Currently this re-compiles the example contracts for each test. This takes a lot of time.

What would you like to see?

We should compile a contract once and cache it for future tests.

What alternatives are there?

Compile all the contracts in advance, which might actually be even faster (since cargo would only need to build the deps once).