test: add coverage for client._addHandle()

`Client.prototype._addHandle()` in the `_debugger` module has conditions
around invalid properties that are not currently tested. This change
adds some minimal unit tests.

test/parallel/test-debugger-client-addhandle.js

@@ -0,0 +1,44 @@
+'use strict';
+
+require('../common');
+const assert = require('assert');
+const Client = require('_debugger').Client;
+
+{
+  const client = new Client();
+  assert.deepStrictEqual(client.handles, {});
+}
+
+{
+  const client = new Client();
+  client._addHandle(null);
+  assert.deepStrictEqual(client.handles, {});
+}
+
+{
+  const client = new Client();
+  client._addHandle('not an object');
+  assert.deepStrictEqual(client.handles, {});
+}
+
+{
+  const client = new Client();
+  client._addHandle({handle: 'not a number'});
+  assert.deepStrictEqual(client.handles, {});
+}
+
+{
+  const client = new Client();
+  const validNoScript = { handle: 6, id: 'foo', type: 'not a script' };
+  client._addHandle(validNoScript);
+  assert.deepStrictEqual(client.handles, { 6: validNoScript });
+  assert.deepStrictEqual(client.scripts, {});
+}
+
+{
+  const client = new Client();
+  const validWithScript = { handle: 5, id: 'bar', type: 'script' };
+  client._addHandle(validWithScript);
+  assert.deepStrictEqual(client.handles, { 5: validWithScript });
+  assert.deepStrictEqual(client.scripts, { bar: validWithScript });
+}