Update end-to-end tests for the graph-based decomposition (#2221)

**Context:**
Added a new test file
`frontend/test/pytest/from_plxpr/test_decompose_transform.py` just for
the graph-based decomposition.

**Description of the Change:**
- Removed decompose-lowering tests from `test_from_plxpr.py`
- Added decompose-lowering tests to `test_decompose_transform.py` 

**Benefits:**
- Better scoping `test_from_plxpr.py` 
- Dedicated test file for decompose-lowering

**Possible Drawbacks:**

**Related GitHub Issues:**

Author: maliasadi

frontend/test/lit/test_decomposition.py

@@ -720,7 +720,7 @@ def rz_ry_rz(phi, theta, omega, wires: WiresLike, **__):
         qml.RY(theta, wires=wires)
         qml.RZ(omega, wires=wires)
 
-    @qml.register_resources({qml.RY: 1, qml.PhaseShift: 1})
+    @qml.register_resources({qml.RY: 1, qml.GlobalPhase: 1})
     def ry_gp(wires: WiresLike, **__):
         """Decomposition of PauliY gate using RY and GlobalPhase gates."""
         qml.RY(np.pi, wires=wires)
@@ -729,7 +729,7 @@ def ry_gp(wires: WiresLike, **__):
     @qml.qjit(target="mlir")
     @partial(
         qml.transforms.decompose,
-        gate_set={"RX", "RZ", "PhaseShift"},
+        gate_set={"RX", "RZ", "GlobalPhase"},
         fixed_decomps={
             qml.RY: rz_rx,
             qml.Rot: rz_ry_rz,

frontend/test/pytest/conftest.py

@@ -59,6 +59,18 @@ def use_capture():
         qml.capture.disable()
 
 
+@pytest.fixture(scope="function")
+def use_capture_dgraph():
+    """Enable capture and graph-decomposition before and disable them both after the test."""
+    qml.capture.enable()
+    qml.decomposition.enable_graph()
+    try:
+        yield
+    finally:
+        qml.decomposition.disable_graph()
+        qml.capture.disable()
+
+
 @pytest.fixture(params=["capture", "no_capture"], scope="function")
 def use_both_frontend(request):
     """Runs the test once with capture enabled and once with it disabled."""