Adjust api_test.js no entry-point expected error message

Since webpack 4 now sets a default entry-point of `./src`, causing
the error message to change for this test. (It doesn't seem worth
creating the file to confirm it works, since from the error message
we can tell the test got far enough that Neutrino correctly didn't
complain about an explicit entry-point not being set.)

packages/neutrino/test/api_test.js

@@ -249,13 +249,13 @@ test('fails when trying to run() a non-registered command', async t => {
   await t.throws(Neutrino().run('non-registered').promise());
 });
 
-test('throws when trying to validate config with no entry point', async t => {
+test('throws when trying to validate config with non-existent entry point', async t => {
   const api = Neutrino();
 
   api.register('build', build);
   const result = api.run('build').promise();
 
   const [err] = await t.throws(result);
 
-  t.true(err.message.includes(`configuration misses the property 'entry'`));
+  t.true(err.includes(`Entry module not found: Error: Can't resolve './src'`));
 });