refer to JSON schema in test definitions

- editors may provide suggestions based on the schema

tests/examples/basic/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-Basic"

tests/examples/cantilever_beam/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-CantileverBeam"

tests/examples/cmdline/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-Cmdline"

tests/examples/cuboid_mesh_stdout/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-CuboidMesh"

tests/examples/dynamic_solver/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-DynamicSolver"

tests/examples/force_element/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-ForceElement"

tests/examples/generalized_nonlinear_solver/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-GeneralizedNonlinearSolver"

tests/examples/input/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-Input"

tests/examples/mesh_generation/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-MeshGeneration"

tests/examples/output/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-Output"

tests/examples/periodic_bc/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-PeriodicBC"

tests/examples/quadratic_tetrahedra/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-QuadraticTetrahedra"

tests/examples/quadratic_triangles/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-QuadraticTriangles"

tests/examples/surface_force/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-SurfaceForce"

tests/examples/timoshenko_beam/definition.json

 {
+  "$schema": "../../schema.json",
   "executable": [
     "examples",
     "ae108-examples-TimoshenkoBeam"