tests: add the NAFEMS LE5 Z-section oracle case

The Z-section cantilever under an end torque as five hex20 blocks (two
flanges, two corner blocks, the web; two elements through every wall):
axial stress at A, on the mid-surface of the lower flange at its free
edge, -109.45 against -108 (2%).

The solid model converges to about -109.7, 1.6% beyond the thin-walled
reference, as ESRD's converged p-version solid (-109.5) does. The README
also records the MITC4 mid-surface shell model of the same problem,
which converges to about -111.3 (3.1%), and why the case is the solid.

Part of #97.

tests/oracle/nafems-le5/fixture.json

@@ -0,0 +1,9 @@
+{
+  "note": "FieldCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "sxx_A": {
+      "band": 1.0945472240551884e-7,
+      "value": -109.45472240551884
+    }
+  }
+}