Add MaunaLoa_CO2.txt to cached files (#1615)

* Add MaunaLoa_CO2.txt

* Reorder cached file list

Author: willschlitzer

pygmt/helpers/testing.py

@@ -167,15 +167,16 @@ def download_test_data():
         "@N00W090.earth_relief_03m_p.nc",
         # Earth seafloor age grids
         "@earth_age_01d_g",
-        "S90W180.earth_age_05m_g.nc"  # Specific grid for 05m test
+        "@S90W180.earth_age_05m_g.nc",  # Specific grid for 05m test
         # Other cache files
         "@EGM96_to_36.txt",
+        "@MaunaLoa_CO2.txt",
+        "@Table_5_11.txt",
         "@fractures_06.txt",
         "@hotspots.txt",
         "@ridge.txt",
         "@mars370d.txt",
         "@srtm_tiles.nc",  # needed for 03s and 01s relief data
-        "@Table_5_11.txt",
         "@test.dat.nc",
         "@tut_bathy.nc",
         "@tut_quakes.ngdc",