check out gtest from remote rather than requiring local installation

[markusgft]

check out gtest from remote rather than requiring local installation. Use approach 4 as described here.

In order to get this done, there are 2 options

• download gtest as external project in all 4 ct - modules. This does not seem clean.
• create a large ct cmake-metaproject and install gtest only once. In this case, all ct modules would share most information, i.e. gtest. It remains to be discussed of this is a good direction to take, since it may affect the independence of the modules in the long term.