Test Requirements, Don't Test Code

A recent post from a DO-178B blog made a powerful statement that just struck me a correct.

The only effective testing mechanism is testing requirements - not code. In fact, the very idea of testing code without a reference to a requirement is ridiculous.

The concept of Testable Requirements has been around for some time in the defense and space business, including a paper from Paul Solomon, connecting Performance Based Earned Value with Testable Requirements.
    So what does it mean when some says "code testing," or in the agile vernacular "write the tests first." Are there requirements - written requirements - for these tests to be "tested" against what?