only agent tests link against gtest

This updates the build such that only agent tests are built against gtest libraries. This should hopefully supersede #911.  

```
08:53 ~/work/cyclus/cyclus [atbuild|…10]$ ldd ~/.local/bin/cyclus_unit_tests | grep test
	libbaseagentunittests.so => /home/gidden/.local/lib/cyclus/libbaseagentunittests.so (0x00007f68ca9f5000)
	libgtest.so => /home/gidden/.local/lib/libgtest.so (0x00007f68ca465000)
08:53 ~/work/cyclus/cyclus [atbuild|…10]$ ldd ~/.local/lib/cyclus/libagents.so | grep test
08:54 ~/work/cyclus/cyclus [atbuild|…10]$ ldd ~/.local/bin/cyclus | grep test
08:55 ~/work/cyclus/cyclus [atbuild|…10]$ 
```