Write tests for the move semantics.

Change-Id: Ie3acd1e714b7536e2c92b51e80653eec824bd7bd
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57432
Reviewed-by: Paul Wankadia <junyer@google.com>
(cherry picked from commit 14d3193228e3b3cfd21094474d64434a4840bae9)
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57433