FLiT tests don't work with relative paths

[mikebentley15]

If a test has a relative path, then the full FLiT run as well as the bisect run will fail since they change to different directories. I think this is a downside to FLiT and should be addressed.

I propose that if a test uses a relative path, then it should be relative to the directory that contains the flit-config.toml and custom.mk files. Likely the easiest solution is to run each executable such that the current directory is always the top-level flit tests directory. Another solution is to find a way to give developers an API that allows them to do relative addressing, which seems harder.

[mikebentley15]

It looks like the FLiT tests work with relative paths for the full FLiT run. But it does not work with Bisect. So looks like the fix here is only with the Bisect Makefile.

However, we should add to the documentation that it is not recommended to use relative paths in the source code since it can make the executables in the results directory not able to be run.

See #50