Avoid interacting with LBFactory singleton in tests

Avoid interacting with LBFactory singleton in tests

Instead test that LBFactory::getLBFactoryClass() does the right thing.

In LBFactory::getLBFactoryClass(), the method now requires a configuration
array as a parameter, and returns the class name as a string.

LBFactory::singleton() now passes in $wgLBFactoryConf into getLBFactoryClass(),
while tests can use test configuration.

Only use of LBFactory::getLBFactoryClass() in core is in LBFactory::singleton().

No extension in gerrit uses getLBFactoryClass() and I can find no use of it
when searching places like Github and Google, so it should be safe to make
such change in LBFactory.

Bug: 59105
Change-Id: I71ae7df16bc5c214b9389125140bca5ce68d274c