policy: fix message for invalid manifest specifier

Add test for invalid manifest specifier and fix the error message
which is missing a space ("singletrailing" instead of
"single trailing").

lib/internal/policy/manifest.js

@@ -136,9 +136,8 @@ class DependencyMapperInstance {
           if (!target) {
             throw new ERR_MANIFEST_INVALID_SPECIFIER(
               this.href,
-              target +
-                ', pattern needs to have a single' +
-                'trailing "*" in target');
+              `${target}, pattern needs to have a single trailing "*" in target`
+            );
           }
           const prefix = target[1];
           const suffix = target[2];

test/fixtures/policy-manifest/invalid.json

@@ -0,0 +1,9 @@
+{
+  "resources": {
+    "./fhqwhgads.js": {
+      "dependencies": {
+          "**": true
+      }
+    }
+  }
+}

test/parallel/test-policy-manifest.js

@@ -0,0 +1,21 @@
+'use strict';
+
+const common = require('../common');
+common.requireNoPackageJSONAbove();
+
+const assert = require('assert');
+const { spawnSync } = require('child_process');
+const fixtures = require('../common/fixtures.js');
+
+const policyFilepath = fixtures.path('policy-manifest', 'invalid.json');
+
+const result = spawnSync(process.execPath, [
+  '--experimental-policy',
+  policyFilepath,
+  './fhqwhgads.js',
+]);
+
+assert.notStrictEqual(result.status, 0);
+const stderr = result.stderr.toString();
+assert.match(stderr, /ERR_MANIFEST_INVALID_SPECIFIER/);
+assert.match(stderr, /pattern needs to have a single trailing "\*"/);