Reorganize interpreter tests based on extensions

[sampsyo]

The interpreter tests are not currently broken down into directories based on the Bril extensions they use. (Except for the mem directory, which dates back to the original Fall 2019 project that implemented it.) We should do that—each extension should get its own directory. That will make it easer to differentially test against other interpreter implementations that don't support all the extensions, such as the Rust interpreter, as discussed in #104 (comment).

[Pat-Lafon]

@sampsyo fixed by #204?

[sampsyo]

Indeed! Thanks!!