Fix Windows tests, verify in CI (#394)

* ci: Test on Windows too

Adds a Windows test run to CI.
Go setup relies on GHA for this
because Hermit doesn't yet support Windows.

* fix(mapper_windows_test): assert.NotNil => assert.True

assert.NotNil does not exist.

* filecontent mapper: Handle error from directory

If we couldn't read because the source is a directory,
override the original error message.

* fix(test): Handle platform-specific "file not found" messages

.github/workflows/ci.yml

@@ -12,9 +12,23 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
       - name: Init Hermit
-        run: ./bin/hermit env -r >> $GITHUB_ENV
+        run: ./bin/hermit env -r >> "$GITHUB_ENV"
       - name: Test
         run: go test ./...
+
+  test-windows:
+    name: Test / Windows
+    runs-on: windows-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+      - name: Setup Go
+        uses: actions/setup-go@v4
+        with:
+          go-version: 1.21
+      - name: Test
+        run: go test ./...
+
   lint:
     name: Lint
     runs-on: ubuntu-latest
@@ -22,6 +36,6 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@v2
       - name: Init Hermit
-        run: ./bin/hermit env -r >> $GITHUB_ENV
+        run: ./bin/hermit env -r >> "$GITHUB_ENV"
       - name: golangci-lint
         run: golangci-lint run