Add tests for load_dataarray to increase code coverage (#1518)

Patches missing code coverage of pygmt.load_dataarray
function wrapped in #1439. Checks that a NetCDF grid
can be read with GMTDataArrayAccessor information loaded,
and that load_dataarray fails when the cache argument is used.

pygmt/tests/test_io.py

@@ -0,0 +1,34 @@
+"""
+Tests for input/output (I/O) utilities.
+"""
+import numpy as np
+import pytest
+import xarray as xr
+from pygmt.helpers import GMTTempFile
+from pygmt.io import load_dataarray
+
+
+def test_io_load_dataarray():
+    """
+    Check that load_dataarray works to read a NetCDF grid with
+    GMTDataArrayAccessor information loaded.
+    """
+    with GMTTempFile(suffix=".nc") as tmpfile:
+        grid = xr.DataArray(
+            data=np.random.rand(2, 2), coords=[[0.1, 0.2], [0.3, 0.4]], dims=("x", "y")
+        )
+        grid.to_netcdf(tmpfile.name)
+        dataarray = load_dataarray(tmpfile.name)
+        assert dataarray.gmt.gtype == 0  # Cartesian grid
+        assert dataarray.gmt.registration == 1  # Pixel registration
+        # this would fail if we used xr.open_dataarray instead of
+        # load_dataarray
+        dataarray.to_netcdf(tmpfile.name)
+
+
+def test_io_load_dataarray_cache():
+    """
+    Check that load_dataarray fails when the cache argument is used.
+    """
+    with pytest.raises(TypeError):
+        _ = load_dataarray("somefile.nc", cache=True)