Create a set of integration test helpers

[Michael-F-Bryan]

Several crates have integration tests which exercise the bindings generated by wasmer-pack. Actually writing the integration test code can be kinda brittle because it relies on external programs (e.g. python and wapm) which need to be installed, and paths tend to change depending on the OS.

It'd be nice to pull all this integration test logic out into a single helper crate that lets you,

• Compile a project to a WAPM package
• Generate bindings for that package
• Set up a Python/Yarn/NPM environment with the bindings installed
• Run a set of integration tests defined in that language
• Cache dependencies from previous runs to avoid re-downloading them

I'm imagining being able to write code like this:

#[test]
fn sha2_integration_tests() -> Result<(), Error> {
  let env = TestEnvironment::for_crate("./path/to/Cargo.toml")?;

  env.python("./my_tests.py")
     .javascript("./my_test.js")
     .typescript("./my_tests.ts")
     .execute()?;
}

[Michael-F-Bryan]

I'm reopening this PR because #112 only implemented the Python side of our integration tests.