Add integration tests

[imiric]

The current testing situation in xk6 is... practically nonexistent. The Go coverage tool reports a measly 1%. :(

Unit testing is difficult with a tool like this as it depends directly on the Go toolchain and the filesystem, though that could be abstracted in a way to make testing easier.

For now integration/E2E tests seem like the path of least resistance, and @na-- has recently added some in the k6 repo.

The goal of this issue is to essentially use those tests as base, add them to this repo and expand them to specifically test xk6 functionality. For example, testing:

• that all variations of the --with option work as expected
• behavior with and without the build argument
• behavior with and without the version argument
• that environment variables work as expected

Since changes in xk6 are much less frequent than in k6, it would be good to have the CI task here run on a schedule, say on a weekly basis, or even daily.

The simple extensions used for tests could also serve as documentation and a good starting point for new contributors, so consider having them in a root examples/ directory.