Be a lot smarter about whether this test passes: instead of assuming

Be a lot smarter about whether this test passes: instead of assuming
that a 2.93 sec audio file will always take 3.1 sec (as it did on the
hardware I had when I first wrote the test), expect that it will take
2.93 sec +/- 10%, and only fail if it's outside of that range.
Compute the expected