Well...that true, if
..they write the test after they write a given function. If they write the test before they write the function, the module tends to work properly for the input that conforms to the expected cases, and tends to fail (gracefully) for everything else...which is sort of what you'd expect from a properly written module.
And one tends to hope that input of the form, "the day before Bob's birthday," is not in the set of expected cases....
jb4
shrub\ufffdbish (Am., from shrub + rubbish, after the derisive name for America's 43 president; 2003) n. 1. a form of nonsensical political doubletalk wherein the speaker attempts to defend the indefensible by lying, obfuscation, or otherwise misstating the facts; GIBBERISH. 2. any of a collection of utterances from America's putative 43rd president. cf. BULLSHIT