Make BaseEngineTest behave as BaseEngineParTest by default

When BaseEngineTest is used in multithreaded unit test it will throw exception explaining that perhaps BaseEngineParTest shoul dbe used instead.
It would be better however to make it default by simply renaming:
BaseEngineTest -> BaseIsolatedEngineTest
BaseEngineParTest -> BaseEngineTest
BaseEngineParTest should be just alias to BaseEngineTest and left for backwards compatibility.

Start by locating BaseEngineTest and BaseEngineParTest and reviewing their usages in the multithreaded unit tests. Confirm that the default test base behaves like the parallel version, that the isolated name remains available, and that the old parallel name still works as an alias.

Written by the indexing model from the issue text.