test: migrate 700_e2e rows to corpus fixtures; retire the carrier

136 rows migrated, 0 already owned, 3 wwstage-failing rows held out.

test/wcc/data/r700_nullable_arr_elems/case.ww

@@ -0,0 +1,24 @@
+//ww:run-exit 201
+// Migrated from 700_e2e row 95.
+package main;
+fn pickptr(b: bool) *i32 = {
+  let x: i32 = 42;
+  if (b) { return &x; };
+  return nil;
+};
+fn main() i32 = {
+  let arr: [3](*i32 | void);
+  arr[0] = nil;
+  arr[1] = pickptr(true);
+  arr[2] = pickptr(false);
+  let s: i32 = 0;
+  let i: i32 = 0;
+  for (i < 3) {
+    match (arr[i]) {
+      case let p: *i32 => s += 1;
+      case => s += 100;
+    };
+    i += 1;
+  };
+  return s;
+};