Add a test to check that FLAGS is deepcopy-able.
Add a test to check that FLAGS is deepcopy-able.

There are some unit tests for deepcopy support, but none that ensure the
flags defined by absl behave correctly.

PiperOrigin-RevId: 305146969
Change-Id: Ib46316634aac60142a99a5163753e565148818ef

abseil/abseil-py,abseil/abseil-py