Allow for skipping Lua tests

From the PHP subclass of Scribunto_LuaEngineTestBase, Lua tests run by
Scribunto_LuaEngineTestBase::testLua() may be skipped by adding an entry
for the test name to $this->skipTests.

From the Lua code run by Scribunto_LuaEngineTestBase::testLua(), tests
may be skipped by calling the "markTestSkipped" function exported by the
TestFramework module.

Also, use this new mechanism to skip certain mw.language tests if
Extension:CLDR or a similar extension is not available.

Bug: 67343
Change-Id: I4f0b15073a84bf2f9d8a5d905c3c960941dd2b71
Merge "Allow for skipping Lua tests"