fixing few more tests: hide nb_users to make tests pass because for some reason the column does appear by default but in the backwardsCompatibilityTest it does not appear there. Also had to manually activate and install plugins in the test case for some reason. Message was:
```
Exception: Failed to update database (errors or warnings found): Array
(
    [warnings] => Array
        (
        )

    [errors] => Array
        (
            [0] => /home/matt/dev/piwik-master/plugins/CustomAlerts/Updates/0.0.2.php:
Error trying to execute the query 'ALTER TABLE `alert` CHANGE `enable_mail` `email_me` BOOLEAN NOT NULL'.
The error was: SQLSTATE[42S02]: Base table or view not found: 1146 Table 'piwik_tests.alert' doesn't exist
        )

    [coreError] =>
    [deactivatedPlugins] => Array
        (
            [0] => CustomAlerts
        )

)

```

  - moved all API URL generation logic & API response handling logic to separate classes
  - created ApiTestConfig class to hold API test options (and documentation for properties)
  - remove unnecessary IntegrationTestCase properties and methods (including apiNotToCall/apiToCall)
  - remove as many unnecessary API test options as possible (including abandonedCarts, hackDeleteRangeArchivesBefore)
  - rename checkRequestResponse to assertApiResponseHasNoError

Fixes #5809, change existing SQL to ignore errors that occur when updates are executed more than once, add test for multiple update and allow updates to specify more than one error code to ignore.

Refs #5800, add namespaces to integration tests in root folder. Also add IntegrationTestCase to a namespace, re-add individual test @groups, move test groups to class level, and rename test files & class names so underscores are not used.

Modified SetupFixture to save SQL dumps of fixtures when needed, make sure OmniFixture can be used w/ SetupFixture, and use SQL dump of OmniFixture in UI tests instead of running OmniFixture every time (so changes in fixtures do not result in changes to screenshots).

Piwik is a free/libre analytics platform. Refs #4455 GNU Package requirement to use free/libre instead of open source

Fixes #4878, re-introduce VisitFrequency Archiver in order to successfully combine pre-2.0 VisitFrequency data with new data.

Enable commented out tests in BackwardsCompatibility1XTest by executing update after sql dump is loaded.

Refs #4878, fix bug in VisitFrequency.addPrePiwik2DataIfNewDataAbsent and add test for multiple periods w/ pre Piwik 2 VisitFrequency data.

Fixes #4878, use pre-piwik 2 data in VisitFrequency API if post-piwik 2 data cannot be found. Commit includes commented out tests for changes.

- Rename *_BaseFixture to Fixture.
- Move DB setup code to Fixture and reuse code in DatabaseTestCase.
- Make IntegrationTestCase::$defaultApiNotToCall an instance var.

Notes:
  - Allow integration tests to compare processed files against expected files of other tests.
  - Allow integration tests to modify apiNotToCall.