Stub C++ version of pgraster test

[strk]

NOTE: this is a much less interesting than the python test

This is a spin-off of #3348, only including the testcase stub.

[strk]

BTW, https://github.com/mapnik/python-mapnik/blob/master/test/python_tests/pgraster_test.py should really be ported to C++ for it to be a good test.

[lightmare]

I think the run function should be local, too. Unit tests are linked into a single executable. Either make it static, or move to unnamed namespace like you did dbname.

[strk]

Also test name needed to be changed. Should work now.