test: package tests for shared imports

[xianzuyang9-blip]

Fixes #7.

Several test modules import shared vendored helpers with absolute imports like from tests.test_compressobj import HAMLET_SCENE. Adding tests/__init__.py makes the local test directory an explicit package, so those imports resolve to this repository instead of depending on namespace-package discovery or being shadowed by another installed tests package.

Verification:

• python -c "import importlib.util; spec=importlib.util.find_spec('tests'); print(spec.origin); print(list(spec.submodule_search_locations)); spec2=importlib.util.find_spec('tests.test_compressobj'); print(spec2.origin)" now resolves tests to this repository's tests/__init__.py.
• git diff --check

I could not run the full pytest suite on this Windows machine because the editable build needs the MSVC link.exe linker for the Rust extension, which is not installed in this environment.