Add infrastructure for reading Python command output

This will be used in the future to make tests/assertions for
Python/Haskell codebase equivalence (e.g. checking that the same
opcodes are defined, etc.).

(Side-note: this might not work in VPATH builds; I'll see how it goes)

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>