fix: add testnetChannelEndpoint()

web3.js/test/testnet.test.js

@@ -0,0 +1,18 @@
+// @flow
+import {testnetChannelEndpoint} from '../src/util/testnet';
+
+test('invalid', () => {
+  expect(() => {
+    testnetChannelEndpoint('abc123');
+  }).toThrow();
+});
+
+test('edge', () => {
+  expect(testnetChannelEndpoint('edge')).toEqual(
+    'https://api.edge.testnet.solana.com',
+  );
+});
+
+test('default', () => {
+  testnetChannelEndpoint(); // Should not throw
+});