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_match/case.ww

@@ -0,0 +1,22 @@
+//ww:run-exit 7
+// Migrated from 700_e2e row 66.
+package main;
+fn lookup(p: *i32, b: bool) (*i32 | void) = {
+  if (b) { return p; };
+  return;
+};
+fn use_arg(r: (*i32 | void)) i32 = {
+  match (r) {
+    case let q: *i32 => return *q;
+    case void => return 99;
+  };
+  return 0;
+};
+fn main() i32 = {
+  let x: i32 = 42;
+  let ok: i32 = use_arg(lookup(&x, true));
+  let no: i32 = use_arg(lookup(&x, false));
+  if (ok != 42) { return 1; };
+  if (no != 99) { return 2; };
+  return 7;
+};