test: resolve false errors about side effects in bash-completion tests

[reubeno]

Brings pass rate of bash completion test suite to 60+%.

[github-actions]

Test Results

    2 files      9 suites   1m 32s ⏱️
  671 tests   671 ✅ 0 💤 0 ❌
1 328 runs  1 328 ✅ 0 💤 0 ❌

Results for commit f56015f.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
clone_shell_object	17.89 μs	18.02 μs	0.13 μs	🟠 +0.73%
eval_arithmetic	0.17 μs	0.17 μs	0.00 μs	⚪ Unchanged
expand_one_string	1.88 μs	1.82 μs	-0.06 μs	⚪ Unchanged
for_loop	22.30 μs	22.32 μs	0.02 μs	⚪ Unchanged
function_call	2.44 μs	2.39 μs	-0.05 μs	⚪ Unchanged
instantiate_shell	56.89 μs	57.48 μs	0.59 μs	🟠 +1.04%
instantiate_shell_with_init_scripts	21855.41 μs	22341.78 μs	486.37 μs	🟠 +2.23%
parse_bash_completion	1659.37 μs	1669.30 μs	9.93 μs	⚪ Unchanged
parse_sample_script	1.76 μs	1.79 μs	0.03 μs	🟠 +1.47%
run_echo_builtin_command	15.03 μs	15.30 μs	0.28 μs	⚪ Unchanged
run_one_external_command	2407.80 μs	2359.55 μs	-48.25 μs	🟢 -2.00%
tokenize_sample_script	2.80 μs	2.74 μs	-0.06 μs	🟢 -2.28%

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
brush-core/src/builtins/set.rs	🟢 77.73%	🟢 78.03%	🟢 0.3%
brush-core/src/sys/unix/signal.rs	🟠 64.41%	🟠 61.02%	🔴 -3.39%
Overall Coverage	🟢 77.98%	🟢 77.97%	🔴 -0.01%

Minimum allowed coverage is 70%, this run produced 77.97%

Test Summary: bash-completion test suite

Outcome	Count	Percentage
✅ Pass	1307	62.24
❗️ Error	226	10.76
❌ Fail	183	8.71
⏩ Skip	369	17.57
❎ Expected Fail	14	0.67
✔️ Unexpected Pass	1	0.05
📊 Total	2100	100.00