support test execution in custom directory

[amcnicho]

Some machines (such as NRAO cvpost nodes or ESO Jenkins perf containers) have smaller allocation of space to filesystems mounted at the default location of asv's runner. This can cause test failures, e.g., OSError: [Errno 28] No space left on device: '/tmp/tmp5fmsc3vi'. Not all users will have easy ability to reconfigure their machines to prevent this class of errors but that should not preclude running benchmarks on those platforms.

Refactoring the setup methods to not copy data locally helps, but not for tasks that produce large volume of output data -- which may eventually be required. Therefore it will be useful to configure asv with explicitly specified test location. Also it would have the tangential benefit of facilitating benchmarks that compare test execution with data on disk locally vs. shared file systems.

The definition of _run_benchmark_single_param includes a parameter cwd that, if accessible through the API, should allow for this.