We need to simplify unit test a little bit by adding paddle_test_util static lib. This is a supporting work to #1003 and #1009

The possible gains could be:

1. For example, in add paddle functions for Matrix ContextProjection APIs #1009,
   add_unittest(ContextProjectionOpTest
   ContextProjectionOpTest.cpp
   ../gserver/tests/TestUtil.cpp)
   could be simplified as add_simple_unittest(ContextProjectionOpTest).

2. In gserver/tests/CMakeLists.txt, we could remove the dependency of TestUtil.cpp in many unit tests.