Fix tests

[stepancheg]

Seems two things changes:

• py_test does not see a module in another src
• Rust no longer accepts d[1, 2] = 3

[stepancheg]

Something is not right. This PR fixes the test on my machine. There must be something different, but I don't understand what and where.

[stepancheg]

Figured out, the problem was with rust, not with go (confused by go in test name). Starlark-rust no longer allows d[1, 2] = 3.

Also, I figured we don't actually check error messages in Rust, because error message in Rust includes code snippet, added a todo.