Add t.helper() to skip helper frames in assertion error locations

Problem

When a test fails inside a helper function, the error location points to the helper's internal line, not the test that called it.

Proposal

Add t.helper() — mark a function as a helper so assertion errors point to its caller instead.

local function assert_foo(actual)
    t.helper()
    t.assert_equals(actual, 'foo')
end

function g.test_foo()
    assert_foo('bar')
    -- error points HERE, not inside assert_foo
end

Works with nested helpers — all helper frames are skipped.

Inspired by Go's t.Helper().

Start by tracing how assertion failures determine their source location and how test calls are represented. Add the t.helper() behavior described in the proposal, then verify that marked helper frames are skipped, including nested helpers, while the calling test location is reported.