Compute project directory as what's activated, not pwd, like ]test does

[NHDaly]

It looks like there's a TODO note here already, but just creating an issue to track it:

RegressionTests.jl/src/RegressionTests.jl

Line 85 in af8d60e

project = pwd(), # run from project directory

Our repo is organized with lots of subpackages, and i was expecting to be able to run this:

julia> Pkg.activate("packages/SubPackage")

julia> import RegressionTests

julia> RegressionTests.test()

but I got:

julia> RegressionTests.test()
ERROR: IOError: open("/Users/nathandaly/work/bench", 0, 0): no such file or directory (ENOENT)

It would be nice to set the default project to be the path of the current_project(), similar to what running Pkg.test() would do.