Update RBC test to use smaller grid size for correctness testing.

Timeouts occurred when running the RBC test with comm. This reduces the grid size for correctness testing to reduce the execution time.

### TODO

* [x] correctness test with default config (`start_test test/studies/rbc/tvandoren/`)
* [x] correctness test with comm = gasnet (verify does not timeout) (`CHPL_COMM=gasnet SSH_SERVERS=chap11 SSH_OPTIONS=-q start_test test/studies/rbc/tvandoren/`)
* [x] correctness test with valgrind (verify does not timeout) (`start_test --valgrind test/studies/rbc/tvandoren/`)