Now, the unit tests are ran twice -- once with POST_BUILD (i.e. default mode)
and again with PRE_TEST (i.e. new discovery mode).

Both modes of setting gtest discovery mode are also tested:
1. Using the global override (i.e. CMAKE_GTEST_DISCOVER_TESTS_DISCOVERY_MODE)
2. Explicitly passing DISCOVERY_MODE in calls to gtest_discover_tests (in GoogleTestDiscoveryTimeout.cmake)

The goal is to show that the new PRE_TEST discovery mode does not break existing behavior
(i.e. should not break POST_BUILD mode) and should also pass the same tests
in the same way.

A few non trivial implementation details worth noting:

1. Refactoring discovery_timeout_test into own project

Originally, I tried doing:

```
run_GoogleTest(POST_BUILD)
run_GoogleTest(PRE_TEST)
```

Without changing the internal structure of run_GoogleTest.

But since discovery_timeout_test is part of the same project as the other tests,
and CTest include files always get evaluat...