Skip long running tests with env var instead of hard-coded variable

[Aariq]

Currently whether long tests are skipped or not is handled by a get_skip_long_tests() in zzz.R that is hard-coded to return TRUE. Might be better to control this with an environment variable, e.g., NPN_RUN_LONG_TESTS and have skip_long_tests() return !Sys.getenv("NPN_RUN_LONG_TESTS")

[Aariq]

Should also explore the possibility that these tests don't need to be downloading so much data. The point is to test the R functions, not the API or the data—maybe tests can be pared down to download a much smaller slice of data?