Integration tests for examples/

[bartlomieju]

To prevent problems like https://github.com/denoland/deno_core/pull/405, we should add some minimal integration tests for our examples/ directory that would assert specific output. We should use the same approach we use in "deno" - ideally sharing the code between the crates: https://github.com/denoland/deno/blob/bac51f66aa89dd9b79a3c4e844423c3a3399ea13/test_util/src/lib.rs#L477-L496