Fix test case names. (#7406)

This fixes a minor issue I found while working on #7291 

The test case names don't match the method being tested. This is
confusing because there is another method called
`merge_single_qubit_moments_to_phxz`, but it's not the one tested here.

Author: codrut3

cirq-core/cirq/transformers/merge_single_qubit_gates_test.py

@@ -221,13 +221,13 @@ def test_merge_single_qubit_moments_to_phxz_deep():
     )
 
 
-def test_merge_single_qubit_moments_to_phxz_global_phase():
+def test_merge_single_qubit_gates_to_phxz_global_phase():
     c = cirq.Circuit(cirq.GlobalPhaseGate(1j).on())
     c2 = cirq.merge_single_qubit_gates_to_phxz(c)
     assert c == c2
 
 
-def test_merge_single_qubit_moments_to_phased_x_and_z_global_phase():
+def test_merge_single_qubit_gates_to_phased_x_and_z_global_phase():
     c = cirq.Circuit(cirq.GlobalPhaseGate(1j).on())
     c2 = cirq.merge_single_qubit_gates_to_phased_x_and_z(c)
     assert c == c2