test: give each test its own temporary directory

[ferdingler]

Description

Twenty-three tests in tests/shell_integration.rs and tests/curl_integration.rs use fixed host paths such as $TMPDIR/lsh_config_test. The grep binary fixture, which 47 tests use, also deletes its directory before it starts. When two test runs share a host, they write, read, and delete the same directories, and random tests fail. On main (ef2fbca), 4 of 6 runs failed when two full suites ran at the same time. A different set of tests failed each time, for example config_file_with_binds_and_creds, bind_direct_write_back, curl_allowed_url_via_toml_config, and several grep_* tests.

This PR adds a small TestDir helper to each of the two files. It creates a new directory named with the test name, the process ID, and a counter, and it removes the directory on drop. A failure to create or remove the directory now fails the test. Before, the tests ignored these errors. The dangling-symlink test now binds a subdirectory, so its escape target stays inside the test directory and outside the mount.

No production code changes. No dependency is added.

Related Issues

None.

Documentation PR

Not applicable.

Type of Change

Other: test reliability.

Testing

• cargo test --workspace --all-targets: 1805 passed, 0 failed. This is the same count as main.

• Collision: three rounds of two full suites at the same time, each with its own CARGO_TARGET_DIR.

  • main: 4 of 6 runs failed (7, 3, 1, and 2 failing tests).
  • This branch: no directory failures in 12 runs. In 2 of the 12 runs, bind_direct_write_back failed because the host file was still empty after its fixed 50 ms sleep. Its directory is unique, so this is a separate timing problem. With the sleep set to 2 s for the experiment only, 6 of 6 runs passed. This PR does not change that test's timing.

• cargo clippy --workspace --all-targets: 14 warnings, the same as main. cargo fmt --check is clean.

• I ran the relevant test suites for the bindings I touched (cargo test --workspace --all-targets, pytest tests/python, npm test)

• If I touched Rust, I ran cargo fmt and cargo clippy

Checklist

• I have read the CONTRIBUTING document
• I have reviewed and understand every line of code in this PR, including any generated by AI tools, and I can explain why it works
• My change is focused and reasonably small; I have split unrelated work into separate PRs
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly (not applicable)
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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