Support for testing subdirectory packages in `IntegrationTest.yml`

[mtfishman]

Currently, the IntegrationTest.yml workflow expects a Github repository as an input, and runs the tests of the top level Julia package of the repository. The current workflow doesn't allow for testing subdirectory packages, for example NDTensors.jl is a subdirectory package living in the ITensors.jl repository (https://github.com/ITensor/ITensors.jl/tree/v0.8.0/NDTensors). It shouldn't be difficult to add support for that kind of setup.

Maybe we could use this notation:

jobs:
  integration-test:
    name: "IntegrationTest"
    strategy:
       matrix:
         repo:
           - 'ITensor/ITensors.jl:NDTensors'

inspired by the syntax of Pkg.add: https://pkgdocs.julialang.org/v1/managing-packages/#Adding-a-package-in-a-subdirectory-of-a-repository.

[mtfishman]

This has been fixed.