Fix tests due to bad input to assemble function (Qiskit#1039)

In Qiskit/qiskit#5298 a bug was fixed to ensure that a singe entry list
passed to `transpile()` would always return a list instead of just a circuit
object. However, in the aer tests there were a couple of places that were
building a list assuming transpile would return a bare circuit object. When
the terra fix merged changing the return type the tests started to fail because
the input to the `assemble()` function was now a list of a list of circuits instead
of the intend list of circuits. This commit fixes this issue so it works with the
corrected terra behavior.

test/terra/backends/qasm_simulator/qasm_fusion.py

@@ -188,7 +188,7 @@ def test_kraus_noise_fusion(self):
                             backend=self.SIMULATOR,
                             basis_gates=noise_model.basis_gates,
                             optimization_level=0)
-        qobj = assemble([circuit],
+        qobj = assemble(circuit,
                         self.SIMULATOR,
                         shots=shots,
                         seed_simulator=1)
@@ -217,7 +217,7 @@ def test_non_kraus_noise_fusion(self):
                             backend=self.SIMULATOR,
                             basis_gates=noise_model.basis_gates,
                             optimization_level=0)
-        qobj = assemble([circuit],
+        qobj = assemble(circuit,
                         self.SIMULATOR,
                         shots=shots,
                         seed_simulator=1)