PackageManagerKernelTestBase::assertResults does not give helpful error message if asserting results are empty fails

Problem/Motivation

because PackageManagerKernelTestBase::assertResults()

// If we did not get an exception, ensure we didn't expect any results.
      $this->assertEmpty($expected_results);

If a test fails because the results are not empty you will just see

Failed asserting that an array is not empty.

which is not very helpful

Steps to reproduce

To force a failed test like what might happen in debugging an issue

1. add return statement to the top of \Drupal\package_manager\Validator\LockFileValidator::validateStagePreOperation so we never add an error
2. Run \Drupal\Tests\package_manager\Kernel\LockFileValidatorTest::testLockFileChanged which expects an error
3. get the error "Failed asserting that an array is not empty."

Proposed resolution

replace

$this->assertEmpty($expected_results);

with

$this->assertValidationResultsEqual([], $expected_results);

which will show the error we were expecting

Remaining tasks

User interface changes

API changes

Data model changes