Issue #1431658 by Albert Volkman, xjm, kid_icarus, ChinggizKhan, rgristroph, connork: Massive API docs cleanup for system tests

Issue #1739994 by pp, das-peter, Gábor Hojtsy: Use the Language class universally instead of stdObj instances.

Issue #1711070 by kid_icarus, tim.plunkett, xjm, droplet, zendoodles and cam8001: Convert tests to use ::$modules property instead of parent::setUp($modules).

Issue #1496542 by cosmicdreams, sun, chx, vasi1186, aspilicious, alexpott, nadavoid, Pedro Lozano, disasm, andypost | heyrocker: Convert site information to config system.

Issue #1561004 by marcingy, aries, yurtboy, Tor Arne Thune: Many tests still reference locale although they only need language module.