Add regression test for AMR w/ high-order SCALARs

test/tests/adaptivity/scalar/scalar_adaptivity.i

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+[Mesh]
+  type = GeneratedMesh
+  dim = 2
+  nx = 10
+  ny = 10
+  xmin = -0.5
+  xmax = 0.5
+  ymin = -0.5
+  ymax = 0.5
+[]
+
+[Variables]
+  [scalar]
+    order = THIRD
+    family = SCALAR
+  []
+  [u]
+    [InitialCondition]
+      type = FunctionIC
+      function = 'x*x+y*y'
+    []
+  []
+[]
+
+[Kernels]
+  [u_dot]
+    type = TimeDerivative
+    variable = u
+  []
+  [c_res]
+    type = Diffusion
+    variable = u
+  []
+[]
+
+[ScalarKernels]
+  [d1]
+    type = ODETimeDerivative
+    variable = scalar
+  []
+[]
+
+[BCs]
+  [Periodic]
+    [all]
+      auto_direction = 'x y'
+      variable = 'u'
+    []
+  []
+[]
+
+[Preconditioning]
+  [SMP]
+    type = SMP
+    full = true
+  []
+[]
+
+[Executioner]
+  type = Transient
+  solve_type = 'PJFNK'
+  petsc_options_iname = '-pc_type -sub_pc_type'
+  petsc_options_value = 'asm         lu     '
+  num_steps = 2
+[]
+
+[Adaptivity]
+  initial_steps = 2
+  max_h_level = 2
+  marker = EFM
+  [Markers]
+    [EFM]
+      type = ErrorFractionMarker
+      coarsen = 0.2
+      refine = 0.8
+      indicator = GJI
+    []
+  []
+  [Indicators]
+    [GJI]
+      type = GradientJumpIndicator
+      variable = u
+    []
+  []
+[]
+
+[Outputs]
+  exodus = true
+[]

test/tests/adaptivity/scalar/tests

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+[Tests]
+  issues = '#12538 #12309'
+  design = 'syntax/Adaptivity/index.md'
+  [./test]
+    type = 'Exodiff'
+    input = 'scalar_adaptivity.i'
+    exodiff = 'scalar_adaptivity_out.e-s002'
+    requirement = "The Adaptivity system shall support high-order SCALAR variables"
+  [../]
+[]