I hit some false positives while working on sth else. This fixes them. Making the testing more robust.

What this does

linopy expressions are dimension-labelled xarray structures, so they inherit
xarray's broadcast order-sensitivity: x + y produces dims (x_dim, y_dim)
while y + x produces (y_dim, x_dim). That difference is not mathematically
meaningful, but xarray's assert_equal is order-sensitive, so
assert_linequal(x + y, y + x) failed.

assert_linequal already normalizes term order before comparing (its
docstring promises "semantically equal"). This aligns dimension order the
same way: a new _align_dim_order helper transposes the second operand to the
first's dimension order when they share a dimension set. If the dimension sets
differ, the expressions are genuinely unequal and the comparison is left
untouched so assert_equal reports the real difference.

Notes

• Touches only linopy/testing.py; adds test/test_testing.py (dim-order
  insensitivity, plus genuinely-different expressions still fail).
• Full test_linear_expression.py + test_quadratic_expression.py pass
  unchanged (333), i.e. nothing was relying on the order-sensitive behavior.

import pandas as pd
from linopy import Model
from linopy.testing import assert_linequal

m = Model()
a = m.add_variables(coords=[pd.Index([0, 1], name="i")], name="a")
b = m.add_variables(coords=[pd.Index([0, 1, 2], name="j")], name="b")

(a + b).data.coeffs.dims   # ('i', 'j', '_term')
(b + a).data.coeffs.dims   # ('j', 'i', '_term')
assert_linequal(a + b, b + a)   # before: AssertionError; after: passes