[benchmarks] Change tests to register using the perftest library directly

Remove some uses of the "fbenchmark" wrapper.  This used to register
tests with two libraries -- gbenchmark and perftest -- but this is no
longer necessary now that we don't support using gbenchmark.

Convert the Fstat test to be a function rather than a class.

Bug: TO-651
Test: /pkgfs/packages/zircon_benchmarks/0/test/zircon_benchmarks
Change-Id: Idcc341b26d08a69ce925484d03bfc516d1ee236d
2 files changed