To begin with, I question the wisdom or usefulness of such
a degree of specialization. Call me "old school" but I respect
more, someone who knows and owns the design from bottom
to top. This sort of "somebody else's problem, because I don't
feel like dealing" leads to dysfunctional design teams and bad
products.
But specifically to your question, testability begins with the
detailed design and a "backend" engineer whose only role and
authority is to faithfully implement a "frontend" design that
lacks the test "backbone" is simply doomed to fail at meeting
testability metrics and actual testability, and be simply one
link in a chain of fail that ends in blown-out test development
schedules and in-production test escapes.
Tools are secondary to responsibility, effort, skill and diligence.
And they will not substitute for decent judgment about how
to get test coverage without (unduly) compromising the other
aspects of design (performance, compactness, reliability, etc.).
Prima donna behaviors don't make for career success in
engineering. Get on the team and get your hands dirty.