Give path to executables in tests

[nmattia]

Describe the problem you are trying to solve

I'm trying to run some of my crate's executables in a test. Currently I have to

1. Figure out the relative path to target (that path changes depending on whether this is a single crate project (./target) or if the crate is being built as part of a workspace (../target)).
2. Figure out whether this is a release or debug build by comparing the modification times of the executable.

Describe the solution you'd like

I would like environment variables that tell me

1. The full path to where the executables are located.
2. (optionally) Whether this is a debug or release build.

Notes

cargo makes sure that the executables are built before the tests are run, so I believe this doesn't go against cargo best-practices to use freshly-built executables in test, but I'm happy to reconsider.

[ehuss]

Thanks for the report! Issue #5758 is already tracking this, so I'm going to close in favor of that. I'm working on a solution, so hopefully will have something soonish.