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_is_as/case.ww

@@ -0,0 +1,18 @@
+//ww:run-exit 7
+// Migrated from 700_e2e row 67.
+package main;
+fn lookup(p: *i32, b: bool) (*i32 | void) = {
+  if (b) { return p; };
+  return;
+};
+fn main() i32 = {
+  let x: i32 = 42;
+  let r1: (*i32 | void) = lookup(&x, true);
+  let r2: (*i32 | void) = lookup(&x, false);
+  let acc: i32 = 0;
+  if (r1 is *i32) { acc += 1; };
+  if (r2 is void) { acc += 2; };
+  let p: *i32 = r1 as *i32;
+  if (*p == 42)   { acc += 4; };
+  return acc;
+};