close stdout/stderr handles in run_shell_cmd and test teardown + remove duplication in toy_build tests

[Flamefire]

Almost every test case starts by getting the path to the test-folder, then to toy-0.0.eb and then reading this file.

Not only makes this harder to read the (relevant part of) the test it also slows it down to read the same file in every test.

Hence I factored those out to file-level constants.

While testing I noticed we don't close the mocked stdout/stderr buffers and added that too

[Flamefire]

Rebased

[boegel]

@Flamefire Needs conflict fixing

[Flamefire]

Rebased to fix

[boegel]

lgtm

[Flamefire]

I noticed there are > 200 similar cases in other test files.
@boegel Would you merge a PR replacing those too? My idea would be to move the constants to test/framework/__init__.py so we can import them with from test.framework import TEST_ECS_DIR, TOY_EC, TOY_EC_TXT in the 18 test files that use them.
Starting many tests with this boilerplate code gets tiresome.

[boegel]

I noticed there are > 200 similar cases in other test files. @boegel Would you merge a PR replacing those too? My idea would be to move the constants to test/framework/__init__.py so we can import them with from test.framework import TEST_ECS_DIR, TOY_EC, TOY_EC_TXT in the 18 test files that use them. Starting many tests with this boilerplate code gets tiresome.

@Flamefire Sure, sounds like a nice cleanup in the tests. (sorry for the late reply, I often overlook comments in merged PRs)

[Flamefire]

Followup done: #5203

300 lines removed