add a way to assert expectations on the output

[Gankra]

While I am increasingly inclined to snapshot testing and therefore #7, I think it would also be reasonable to assert that certain frames do or don't appear, similar to the CHECK-NEXT machinery in llvm's tests?

[Gankra]

The big issue with this is of course we are at the whims of compiler optimizations / toolchain updates for many of our core inputs. How stable is a backtrace across different compilers? How should we balance pinning toolchains vs testing the bleeding edge?