Issue 32 On modules page, hide modules that are required_by /Drupal/ ins...

Apologies about the delay, got busy on other projects so here is some lunchtime effort.

The test "Assert that core required modules cannot be disabled" works if the module is listed, but fails when the project is hidden. 

This update has the original changes I made and also removed the test which has this assertion.
```
$this->assertFieldByXPath("//input[@name='$field_name' and @disabled='disabled' and @checked='checked']", '', t('Field @name was disabled and checked.', array('@name' => $field_name)));
```

If there is another test we need to do, let go with it. At this point I believe that the hidden module testing should be handled by the existing tests as there is no form field on the module page.

Another caveat, the Options module is still enabled, due to taxonomy being enabled by default, but Drupal, er, I mean Backdrop does not depend on it explicitly.

Here are the test messages for modules and system only - did not run all the tests:
"RESULTS
2811 passes, 0 fails, 0 exceptions, and 632 debug messages"