test: bring up macos testing

[reubeno]

• Includes proposed fix for pipelines frequently encounter unexpected SIGSTP on macOS #151 .
• Adjusts some tests (with some TODOs) to get to a good baseline across Linux and macOS; we can improve coverage, debug those TODOs from there.

[github-actions]

Performance Benchmark Report

Benchmark name	Baseline (μs)	Test/PR (μs)	Delta (μs)	Delta %
expand_one_string	3.63 μs	3.63 μs	0.00 μs	⚪ Unchanged
instantiate_shell	59.82 μs	60.54 μs	0.72 μs	⚪ Unchanged
instantiate_shell_with_init_scripts	25082.41 μs	25077.54 μs	-4.87 μs	⚪ Unchanged
parse_bash_completion	5568.50 μs	5583.18 μs	14.68 μs	⚪ Unchanged
parse_sample_script	8.97 μs	8.95 μs	-0.02 μs	⚪ Unchanged
run_echo_builtin_command	92.56 μs	91.47 μs	-1.09 μs	⚪ Unchanged
run_one_builtin_command	113.04 μs	110.80 μs	-2.24 μs	⚪ Unchanged
run_one_external_command	1896.63 μs	1783.12 μs	-113.52 μs	🟢 -5.99%
run_one_external_command_directly	945.20 μs	945.87 μs	0.67 μs	⚪ Unchanged

Code Coverage Report: Only Changed Files listed

Package	Coverage
Overall Coverage	🟢 76.53%

Minimum allowed coverage is 70%, this run produced 76.53%

[github-actions]

Test Results

 24 files  142 suites   53s ⏱️
423 tests 411 ✅ 12 💤 0 ❌
850 runs  822 ✅ 28 💤 0 ❌

Results for commit c4e3d23.

♻️ This comment has been updated with latest results.