benchmark: Support passing v8 flags to benchmarks

The better to test --use-strict effects on performance.

(Spoiler: it has no measurable effect on performance.)
Merge remote-tracking branch 'ry/v0.10'
benchmark: Fix execArgv handling

Bug in 01f3b46 causes the same benchmark to be run repeatedly.

Not so useful for the compare scripts.
Merge remote-tracking branch 'ry/v0.10'
test: speed up simple/test-fs-watch

Don't wait a full second before starting the watcher, 10 ms ought to be
more than enough time.  Reduces running time from 1250 ms to 250 ms on
my system.