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_def_sentinel_err/case.ww

@@ -0,0 +1,30 @@
+//ww:run-exit 19
+// Migrated from 700_e2e row 96.
+package main;
+type error = str;
+def eEOF: error = "eof";
+def eShortRead: error = "short read";
+fn read(n: i64) (i64 | error) = {
+  if (n < 0) { return eEOF; };
+  if (n == 0) { return eShortRead; };
+  return n + 1;
+};
+fn main() i32 = {
+  let r0: (i64 | error) = read(0);
+  let r1: (i64 | error) = read(-1);
+  let r2: (i64 | error) = read(5);
+  let acc: i32 = 0;
+  match (r0) {
+    case let v: i64 => acc += 100;
+    case let e: error => acc += e.len: i32;
+  };
+  match (r1) {
+    case let v: i64 => acc += 100;
+    case let e: error => acc += e.len: i32;
+  };
+  match (r2) {
+    case let v: i64 => acc += v: i32;
+    case let e: error => acc += 100;
+  };
+  return acc;
+};