Skip level 3 in tests

The level 3 preset passmanager by default runs 2q block collection which
is then synthesized into a unitary. As long as we're running that in the
pass manager it's not really possible right now to preserve input
unitaries through the transpilation. A potential follow on PR could
potential skip the synthesis pass if 'unitary' is in the basis so the
optimization pass would just collect 2q blocks into a unitary and pass
that to the backend directly. However, that's not in scope for this PR
(and it's not clear if we want that or not).

test/python/transpiler/test_preset_passmanagers.py

@@ -99,7 +99,9 @@ def test_level0_keeps_reset(self):
         result = transpile(circuit, basis_gates=None, optimization_level=0)
         self.assertEqual(result, circuit)
 
-    @combine(level=[0, 1, 2, 3], name="level{level}")
+    # Level 3 is skipped because the 2q synthesis optimization pass decomposes
+    # the unitary
+    @combine(level=[0, 1, 2], name="level{level}")
     def test_unitary_is_preserved_if_in_basis(self, level):
         """Test that a unitary is not synthesized if in the basis."""
         qc = QuantumCircuit(2)