test: accommodate slight triangle grid variations (#2757)

Triangle can give slightly different results depending on platform and compiler

Author: wpbonelli

autotest/test_grid.py

@@ -2,6 +2,7 @@
 import re
 import warnings
 from contextlib import nullcontext
+from platform import system
 from warnings import warn
 
 import matplotlib
@@ -1492,17 +1493,9 @@ def test_voronoi_grid(request, function_tmpdir, grid_case):
     name = request.node.name.replace("/", "_").replace("\\", "_").replace(":", "_")
     ncpl, vor, gridprops, grid = grid_case()
 
-    # TODO: debug off-by-3 issue
-    #  could be a rounding error as described here:
-    #  https://github.com/modflowpy/flopy/issues/1492#issuecomment-1210596349
-
-    # ensure proper number of cells
-    almost_right = ncpl == 538 and gridprops["ncpl"] == 535
-    if almost_right:
-        warn("off-by-3")
-
     # ensure that all cells have 3 or more points
     invalid_cells = [i for i, ivts in enumerate(vor.iverts) if len(ivts) < 3]
+    assert len(invalid_cells) == 0
 
     # make a plot including invalid cells
     fig = plt.figure(figsize=(10, 10))
@@ -1512,10 +1505,12 @@ def test_voronoi_grid(request, function_tmpdir, grid_case):
     ax.plot(grid.xcellcenters[invalid_cells], grid.ycellcenters[invalid_cells], "ro")
     plt.savefig(function_tmpdir / f"{name}.png")
 
-    assert ncpl == gridprops["ncpl"] or almost_right
-    assert len(invalid_cells) == 0, (
-        f"The following cells do not have 3 or more vertices.\n{invalid_cells}"
-    )
+    # ensure proper number of cells. the grid may be slightly different
+    # on different platforms depending which compiler triangle is built
+    # with, so allow small variations.
+    tol = 10 if system() == "Windows" else 3
+    assert abs(ncpl - gridprops["ncpl"]) <= tol
+    assert len(invalid_cells) == 0
 
 
 @pytest.fixture