Add expm1 test

pystencils_tests/test_sympy_optimizations.py

+import sympy as sp
+
+import pystencils
+
+
+def test_sympy_optimizations():
+    for target in ('cpu', 'gpu'):
+        x, y, z = pystencils.fields('x, y, z:  float32[2d]')
+
+        # Triggers Sympy's expm1 optimization
+        assignments = pystencils.AssignmentCollection({
+            x[0, 0]: sp.exp(y[0, 0]) - 1
+        })
+
+        ast = pystencils.create_kernel(assignments, target=target)
+        code = str(pystencils.show_code(ast))
+        assert 'expm1(' in code