Replace Check by GTest
Add an empty GTest skeleton
Shorten generator function names
Don't expose cgraph.h header

Exposing the header leads to issues later on of mismatched declarations
for functions such as memcmp().
A first sanity check test
Sequential: Remove unused attributes
Graph: Remove unused graph_test binary
Parallel2D: Mark main args unused

This gets rid of the last compiler warnings, which allows us to enable
-Werror in the following commit.
Enable -Werror