[sampleprof] Drop test dependency on the string hash func (NFC)

The SampleProfWriter emits function information in an order determined
by the string hash function. The situation is a bit brittle, because
changing the hash function can break the tests.

Instead of sorting the function samples to get a relaible ordering (that
might be too expensive), make the tests not depend on a particular
ordering of function samples.

Differential Revision: https://reviews.llvm.org/D32516

llvm-svn: 301419