Move util/flags.h into the testing target.
Move util/flags.h into the testing target.

Change-Id: Ic3cf9db8ba2721fc2fc3addbf5f39386590dd93f
Reviewed-on: https://code-review.googlesource.com/c/re2/+/46817
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2