add unit tests to preview schema command

internal/cmd/preview-test/composable-schema-invalid-root.zed

@@ -0,0 +1,6 @@
+import "./composable-schema-user.zed"
+
+definition resource {
+  relation and: user
+  permission viewer = and
+}

internal/cmd/preview-test/composable-schema-root.zed

@@ -0,0 +1,6 @@
+import "./composable-schema-user.zed"
+
+definition resource {
+  relation user: user
+  permission view = user
+}

internal/cmd/preview-test/composable-schema-user.zed

@@ -0,0 +1 @@
+definition user {}

internal/cmd/preview.go

@@ -47,7 +47,7 @@ var schemaCompileCmd = &cobra.Command{
 	Write to stdout:
 		zed preview schema compile root.zed
 	Write to an output file:
-		zed preview schema compile --out compiled.zed
+		zed preview schema compile root.zed --out compiled.zed
 	`,
 	ValidArgsFunction: commands.FileExtensionCompletions("zed"),
 	RunE:              schemaCompileCmdFunc,

internal/cmd/preview_test.go

@@ -0,0 +1,69 @@
+package cmd
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	zedtesting "github.com/authzed/zed/internal/testing"
+)
+
+func TestPreview(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]struct {
+		files     []string
+		out       string
+		expectErr string
+		expectStr string
+	}{
+		`file_not_found`: {
+			files: []string{
+				filepath.Join("preview-test", "nonexistent.zed"),
+			},
+			expectErr: `no such file or directory`,
+		},
+		`happy_path`: {
+			files: []string{
+				filepath.Join("preview-test", "composable-schema-root.zed"),
+			},
+			expectStr: `definition user {}
+
+definition resource {
+	relation user: user
+	permission view = user
+}
+`,
+		},
+		`cannot_be_compiled_because_using_reserved_keyword`: {
+			files: []string{
+				filepath.Join("preview-test", "composable-schema-invalid-root.zed"),
+			},
+			expectErr: "line 4, column 12: Expected identifier, found token TokenTypeKeyword",
+		},
+	}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			require := require.New(t)
+
+			tempOutFile := filepath.Join(t.TempDir(), "out.zed")
+			cmd := zedtesting.CreateTestCobraCommandWithFlagValue(t,
+				zedtesting.StringFlag{FlagName: "out", FlagValue: tempOutFile})
+
+			err := schemaCompileCmdFunc(cmd, tc.files)
+			if tc.expectErr == "" {
+				require.NoError(err)
+				tempOutString, err := os.ReadFile(tempOutFile)
+				require.NoError(err)
+				require.Equal(tc.expectStr, string(tempOutString))
+			} else {
+				require.Error(err)
+				require.Contains(err.Error(), tc.expectErr)
+			}
+		})
+	}
+}