Fix data provider in FixturesTest

[donquixote]

For newer versions of phpunit, data provider methods should be public.

Also, the value keys in data provider records should match parameter names, OR they should simply be parameter lists instead of associative arrays.

Steps to reproduce:

./vendor/bin/phpunit tests/Util/FixturesTest.php --display-all-issues --stop-on-deprecation

We get

1) BigBlueButton\Util\FixturesTest::testStructureOfFixturesIsStillUpToDate
* Data Provider method BigBlueButton\Util\FixturesTest::xmlFileToFunctionMapping() is not public

* Data Provider method BigBlueButton\Util\FixturesTest::xmlFileToFunctionMapping() is not static

/****/tests/Util/FixturesTest.php:114

and multiple of these:

2) BigBlueButton\Util\FixturesTest::testStructureOfFixturesIsStillUpToDate with data set "case01_api_version" ('getApiVersion', 'api_version.xml', true, '', null)
Providing invalid named argument $function for method BigBlueButton\Util\FixturesTest::testStructureOfFixturesIsStillUpToDate() is deprecated and will not be supported in PHPUnit 11.0..

/****/tests/Util/FixturesTest.php:114

[donquixote]

I think $method is more suitable than $function or $requestFunction, because it is, in fact, a method and not a function.