extra::test: handle slow benchmarks more gracefully.

This makes sure we always run benchmarks even if they are predicted to
take a long time, so that we have some non-zero time to display
(although the error bars may be huge for particularly slow benchmarks).

Fixes #9532.
auto merge of #10952 : huonw/rust/1ms-bench, r=cmr

This makes sure we always run benchmarks even if they are predicted to
take a long time, so that we have some non-zero time to display
(although the error bars may be huge for particularly slow benchmarks).

Fixes #9532.