Allow passing of test size to the generated test target

[easontm]

This is mostly just a cleanliness request for runtime, but I would like to be able to specify the expected size for the generate_requirements_test target so I can avoid the following bazel test log warning:

  WARNING: //<module>:generate_requirements_test: Test execution time (5.2s excluding execution overhead) outside of range for MODERATE tests. Consider setting timeout="short" or size="small".