tests: add NAFEMS LE2 and LE3 shell oracle cases

LE2, the cylindrical shell bending patch test, in its two load cases
(edge moment; pressure with the balancing edge traction) on an 8 × 8
MITC4 patch: tangential stress at E 59.936 and 59.904 against 60.

LE3, the hemispherical shell with point loads, on a 32 × 32
longitude-latitude mesh (planar quad4 cells, tri3 at the pole): radial
displacement at A 0.18418 against 0.185.

Both meshes are written node by node from the generator each README
lists, with the refinement that supports its tolerance. Targets are
transcribed from the reproductions of TNSB Rev. 3 in the Abaqus 2016
Benchmarks Guide and the ESRD StressCheck NAFEMS guide.

Part of #97.

tests/oracle/nafems-le2/fixture.json

@@ -0,0 +1,13 @@
+{
+  "note": "FieldCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "stt_E_moment": {
+      "band": 6.000000000000001e-8,
+      "value": 59.93576769689798
+    },
+    "stt_E_pressure": {
+      "band": 6.000000000000001e-8,
+      "value": 59.90367733652293
+    }
+  }
+}