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_strings_byteindex/case.ww

@@ -0,0 +1,19 @@
+//ww:run-exit 10
+// Migrated from 700_e2e row 100.
+package main;
+import strings;
+fn pick(r: (i32 | void), miss: i32) i32 = {
+  match (r) {
+  case let i: i32 => return i;
+  case void => return miss;
+  };
+  return 0;
+};
+fn main() i32 = {
+  let s: str = "hello, world";
+  let i1: i32 = pick(strings.byteindex(s, ','), -1);
+  let i2: i32 = pick(strings.byteindex(s, 'z'), -1);
+  let i3: i32 = pick(strings.byteindex(s, "world"), -1);
+  let i4: i32 = pick(strings.byteindex(s, "nope"), -1);
+  return i1 + i2 + i3 + i4;
+};