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_os_trywrite/case.ww

@@ -0,0 +1,20 @@
+//ww:run-exit 10
+// Migrated from 700_e2e row 97.
+package main;
+import os;
+fn main() i32 = {
+  let buf: [3]u8;
+  buf[0] = 88: u8;
+  let ok: (i64 | os.oserror) = os.trywrite(1, buf.ptr, 1u64);
+  let bad: (i64 | os.oserror) = os.trywrite(999: i32, buf.ptr, 1u64);
+  let acc: i32 = 0;
+  match (ok) {
+    case let n: i64 => acc += n: i32;
+    case let e: os.oserror => acc += -100;
+  };
+  match (bad) {
+    case let n: i64 => acc += -100;
+    case let e: os.oserror => acc += (- (e: i64)): i32;
+  };
+  return acc;
+};