[tests] Fixup return value for fat_extend_chain

The tests were still using 'bool' for the return value
[tests] Added speed tests under "make bench"

They are under a separate makefile target because (a) they should
be compiled WITH optimization (the unit tests are compiled without)
and (b) they should not be counted for coverage.
[perf] Measure benchmarks without -tickcounter

The results in msec take a bit longer but are easier to interpret.
[perf] Measure fat_extend_chain in the harder case

Extending a directory that's next to unallocated space is optimized
in fat.cpp. Added a benchmark that alternates extending two directories
so that this optimization is not available.
[perf] Tune the measurement of fat_extend_chain