Add unit tests for `fetchGDB()` SSURGO File Geodatabase related functions

[brownag]

Currently fetchGDB() and related low-level functions (get_legend_from_GDB(), get_mapunit_from_GDB(), and get_component_from_GDB()) are untested.

• Add a minimal gNATSGO-style Geodatabase to https://github.com/brownag/soilDBdata for use in unit testing
  • Do we need to test different variants of gNATSGO? Possibly not as the current functions have relatively simple behavior and target standard SSURGO tables
    • Version: "2.0" vs. "traditional"
    • Optional elements such as Valu1, DominantComponent tables, or "Concise gSSURGO"?
• Add tests for fetchGDB()
  • Runs without error with default arguments against whole database
  • Handling of 0-row results
  • Check result of childs=TRUE