Fix msan's use_of_uninitialized_value in UprobesUnwindingVisitorTest (#3383)
Fix msan's use_of_uninitialized_value in UprobesUnwindingVisitorTest (#3383)

We were using `make_unique_for_overwrite` to fill fields of test events knowing
that `MockUprobesReturnAddressManager` wouldn't use the their content.

Bug: http://b/220910711

Test: Run the unit tests locally.
google/orbit,google/orbit,google/orbit,google/orbit