Add `Conditional` wrapper to gtest
This follows an initial proposal for an 'EqIff` matcher. `Conditional` was considered more precise as an EqIff() matcher may suffer from `Iff` not being universally understood.
PiperOrigin-RevId: 383407665
Implement 'Contains(e).Times(n)' matcher modifier which allows to test for arbitrary occurrences including absence with Times(0).
PiperOrigin-RevId: 382210276