[Benchmark] add test

[duncan020313]

Add the test benchmark, including its specification, implementation, proof, and GNU comparison tests, within the scope below.

Question	Answer
Where is the GNU code?	test.c: posixtest / unary_operator / binary_operator
What must I implement?	String -n/-z and =/!=; integer -eq/-ne/-lt/-le/-gt/-ge; !, parentheses, -a/-o and GNU argument-count precedence.
What must the spec guarantee?	A declarative expression grammar and rules based on the number of arguments, decimal integer values and Boolean truth; syntax errors distinct from false.
Which APIs can I use?	CliPlan/PlanArgv override, output outcomes and diagnostics; expression rules without IO.
What is outside this task?	Filesystem, identity/access, terminal predicates and the separate [ command.

Source paths in the table are relative to coreutils/src/.

See implementation notes, common implementation rules, and the contribution workflow.