[gtest] Disallow diagnostics in unit tests.
[gtest] Disallow diagnostics in unit tests.

Add a diagnostic handler to the object library ROOTUnitTestSupport. This
handler will be installed for all google tests, and will generate
failures for every diagnostic messages received.
If this handler generates failures, use ROOTUnitTestSupport to catch
expected diagnostics before they reach this handler.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root