Refs #5800, add namespaces to all Fixture classes.

...Variables/tests/Fixtures/VisitWithManyCustomVariables.php

...mplePlugin/tests/fixtures/SimpleFixtureTrackFewVisits.php

...ghts/tests/Fixtures/SomeVisitsDifferentPathsOnTwoDays.php

tests/PHPUnit/Fixture.php

tests/PHPUnit/Fixtures/FewVisitsWithSetVisitorId.php

tests/PHPUnit/Fixtures/InvalidVisits.php

tests/PHPUnit/Fixtures/ManySitesImportedLogs.php

...PHPUnit/Fixtures/ManySitesImportedLogsWithXssAttempts.php

tests/PHPUnit/Fixtures/ManyVisitsWithGeoIP.php

...s/PHPUnit/Fixtures/ManyVisitsWithMockLocationProvider.php

...t/Fixtures/ManyVisitsWithSubDirReferrersAndCustomVars.php

tests/PHPUnit/Fixtures/OmniFixture.php

tests/PHPUnit/Fixtures/OneVisitSeveralPageViews.php

tests/PHPUnit/Fixtures/OneVisitWithAbnormalPageviewUrls.php

tests/PHPUnit/Fixtures/OneVisitorTwoVisits.php

tests/PHPUnit/Fixtures/SomeVisitsAllConversions.php

...tures/SomeVisitsCustomVariablesCampaignsNotHeuristics.php

...PUnit/Fixtures/SomeVisitsManyPageviewsWithTransitions.php

tests/PHPUnit/Fixtures/SomeVisitsWithLongUrls.php

...s/PHPUnit/Fixtures/SomeVisitsWithNonUnicodePageTitles.php