Verification test of nonlinear FV with BDF2

Author: lindsayad

test/tests/time_integrators/bdf2/fv.i

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+[Mesh]
+  type = GeneratedMesh
+  dim = 2
+  nx = 10
+  ny = 10
+[]
+
+[Variables]
+  [u]
+    type = MooseVariableFVReal
+  []
+[]
+
+[FVKernels]
+  [timeu]
+    type = FVTimeKernel
+    variable = u
+  []
+  [diffu]
+    type = FVDiffusion
+    variable = u
+    coeff = 1
+  []
+  [forceu]
+    type = FVBodyForce
+    variable = u
+    function = force
+  []
+[]
+
+[Functions]
+  [exact]
+    type = ParsedFunction
+    expression = 't^3*x*y'
+  []
+  [force]
+    type = ParsedFunction
+    expression = '3*x*y*t^2'
+  []
+[]
+
+[FVBCs]
+  [allu]
+    type = FVFunctionDirichletBC
+    function = exact
+    variable = u
+    boundary = 'left right top bottom'
+  []
+[]
+
+[Executioner]
+  type = Transient
+  solve_type = 'NEWTON'
+  petsc_options_iname = '-pc_type'
+  petsc_options_value = 'hypre'
+  dt = 1
+  end_time = 3
+  scheme = 'bdf2'
+[]
+
+[Postprocessors]
+  [L2u]
+    type = ElementL2Error
+    function = exact
+    variable = u
+  []
+[]
+
+[Outputs]
+  csv = true
+[]