internal: Fix providercache test failures on macOS
internal: Fix providercache test failures on macOS

For reasons that are unclear, these two tests just started failing on
macOS very recently. The failure looked like:

    PackageDir: strings.Join({
      "/",
+     "private/",
      "var/folders/3h/foobar/T/terraform-test-p",
      "rovidercache655312854/registry.terraform.io/hashicorp/null/2.0.0",
      "/windows_amd64",
    },

Speculating that the macOS temporary directory moved into the /private
directory, I added a couple of EvalSymlinks calls and the tests pass
again.

No other unit tests appear to be affected by this at the moment.

rnaveiras/terraform,tommynsong/terraform,apparentlymart/terraform,apparentlymart/terraform,tommynsong/terraform,rnaveiras/terraform,pdecat/terraform,apparentlymart/terraform,apparentlymart/terraform,hashicorp/terraform,ewbankkit/terraform,ewbankkit/terraform,pdecat/terraform,pdecat/terraform,hashicorp/terraform,tommynsong/terraform,apparentlymart/terraform,rnaveiras/terraform,pdecat/terraform,hashicorp/terraform,ewbankkit/terraform,ewbankkit/terraform