Speed-test / benchmark TODOs

[oschaaf]

#2029 relocated files, and punted on two observed issues:

• //benchmark/pagespeed/kernel/cache has its speed tests disabled: they leak and crash. This warrants looking into.
• Should file two issues //pagespeed/net/instaweb/htmlparse/testdata doesn't exist, while two of the benchmarks seem to require that as an input. Looks like some test data might have gone missing.