Support separate project file in `test/Project.toml`

[avik-pal]

Currently having this instead of specifying extras throws the following error:

Reactant: Error During Test at /......../Reactant.jl/test/runtests.jl:47
  Got exception outside of a @test
  trying to activate test environment of an unnamed project
Test Summary: | Error  Total  Time
Reactant      |     1      1  8.0s
ERROR: LoadError: Some tests did not pass: 0 passed, 0 failed, 1 errored, 0 broken.
in expression starting at /......../Reactant.jl/test/runtests.jl:47
ERROR: Package Reactant errored during testing

[avik-pal]

Seems like this is an issue if we provide the path directly via @__DIR__, passing the package name solves this issue.