workflows/checks: Run tests for all modules in this repository

A few of the packages in this repository are separated into their own
Go modules to reflect boundaries of code ownership and/or boundaries with
significantly different external Go module dependencies.

However, that means that the "./..." pattern in a Go command run at the
root doesn't include those packages. The tests in the remote backends are
mostly disabled in our unit test context because they require credentials
for external network services, but nonetheless it's still useful to
visit them and check if their packages can even compile.

Therefore we'll now visit each of the modules and run tests for all
packages in each one. This restores the coverage we had before the module
split, with the only slight difference being that the remote backend tests
now run only after all of the main Terraform tests, rather than being
interleaved with Terraform's own packages as before. Since we're only
visiting them to see if they compile anyway, this doesn't seem like a big
deal.

.github/workflows/checks.yml

@@ -60,7 +60,10 @@ jobs:
 
       - name: "Unit tests"
         run: |
-          go test ./...
+          # We run tests for all packages from all modules in this repository.
+          for dir in $(go list -m -f '{{.Dir}}' github.com/hashicorp/terraform/...); do
+              (cd $dir && go test "./...")
+          done
 
   race-tests:
     name: "Race Tests"