Add support for plotting a subset of outputs

reframe_tests/reframe_tests.py

@@ -293,6 +293,7 @@ class TestSingleCellPml(GprMaxRegressionTest):
     tags = {"test", "serial", "geometery", "box", "pml"}
     sourcesdir = "src/pml_tests"
     model = parameter(["single_cell_pml_2d"])
+    rx_outputs = ["Hx"]
 
 
 @rfm.simple_test
@@ -302,3 +303,4 @@ class TestSingleCellPmlMpi(GprMaxMPIRegressionTest):
     mpi_layout = parameter([[2, 2, 1], [3, 3, 1]])
     serial_dependency = TestSingleCellPml
     model = serial_dependency.model
+    rx_outputs = ["Hx"]