Add tests for MatrixGate over 6 qubits (#1041)

Verifiy that `_translate_MatrixGate` and
`QSimSimulator.compute_amplitudes` raise `NotImplementedError` when
provided a 7-qubit `MatrixGate`.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: mhucka

qsimcirq_tests/qsimcirq_test.py

@@ -18,6 +18,7 @@
 import sympy
 
 import qsimcirq
+import qsimcirq.qsim_circuit
 
 
 class NoiseTrigger(cirq.Gate):
@@ -128,6 +129,26 @@ def _unitary_(self):
         qsimSim.compute_amplitudes(cirq_circuit, bitstrings=[0b0, 0b1])
 
 
+def test_translate_matrix_gate_too_big():
+    gate = cirq.MatrixGate(np.eye(128))
+    with pytest.raises(
+        NotImplementedError, match="only up to 6-qubit gates are supported"
+    ):
+        qsimcirq.qsim_circuit._translate_MatrixGate(gate)
+
+
+def test_simulate_matrix_gate_too_big():
+    qubits = cirq.LineQubit.range(7)
+    gate = cirq.MatrixGate(np.eye(128))
+    circuit = cirq.Circuit(gate.on(*qubits))
+
+    qsim_sim = qsimcirq.QSimSimulator()
+    with pytest.raises(
+        NotImplementedError, match="only up to 6-qubit gates are supported"
+    ):
+        qsim_sim.compute_amplitudes(circuit, bitstrings=[0b0, 0b1])
+
+
 def test_cirq_giant_identity():
     # Pick qubits.
     a, b, c, d, e, f, g, h = [