recipe-tester: support test-only deps via tests/requirements.txt

[ndonkoHenri]

Motivation

The recipe-tester app installs flet + pytest + <recipe>, so the only third-party packages available on-device are the recipe's own Requires-Dist. That's fine for most recipes (numpy etc. come in transitively), but a zero-runtime-dep recipe can't exercise its integration paths on-device at all.

What changed

• stage_recipe.sh: after staging tests, an optional recipes/<name>/tests/requirements.txt (one PEP 508 spec per line; blanks and #-comments skipped) is expanded into the generated pyproject.toml.
• pyproject.toml.tpl: new __TEST_DEPS__ token line inside the dependencies array (replaced with zero or more dep lines — absent file emits nothing).
• README.md: documents the convention, incl. the constraint that each test dep must be installable for the mobile target (pure-Python from PyPI, or a recipe already published on pypi.flet.dev / seeded via prebuild_recipes) — the same constraint a real app faces — and the guidance to prefer importorskip for genuinely-optional integrations.

Implementation notes

• Template expansion is a bash line-loop with printf '%s' rather than sed: PEP 508 markers legitimately contain double quotes (numpy; python_version >= "3.10"), which have no safe portable sed-RHS escaping (BSD vs GNU).
• Deps are emitted as TOML literal (single-quoted) strings so markers pass through verbatim; a single quote inside a spec is rejected with an explicit ::error::.
• Token matches are exact-line, so template comments that merely mention a token pass through unexpanded.
• Empty-array expansion is guarded (${arr[@]+...}) for macOS bash 3.2 under set -u, so the script keeps working for local devs on the system bash.
• No behavior change when the file is absent: generated deps are identical to before (verified by TOML-parsing the output for a recipe without the file).